Here is a reference value of the maximum number of troops per march (march size) for each keep level. (Keep levels 26 to 40).

Note – This is NOT A PERFECTLY ACCURATE maximum value.
The reason for this is that there are factors (below) that can increase your march size regardless of your keep level.

• Monarch Gear
• VIP
• Rank
• etc.

Stating the maximum values for these will not be practical for many people, as they are quite rare cases.

Therefore, these are listed based on my personal assumption that this is how many people are likely to be at this keep level.

Note that there is room to raise the number of march sizes in this article further.

Legend: k30 = keep level 30

k26 – 298,200

When Using Item

• 50% -> 353,200  (+55,000)
• 100% -> 408,200  (+110,000)
• 200% -> 518,200  (+220,000)

Breakdown

• Rally Spot Lv26 (110,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 10/20 (+25,000)
• Research – Supreme Coordination 0/25 (+0) * Requires Academy Lv28
• Research – Prestige 0/20 (+0) * Requires Academy Lv31
• Research – Adv Prestige 0/25 (+0) * Requires Academy Lv33
• VIP10 (+10,000)
• Rank – Earl (+20,000)
• Monarch Gear – War Horn Lv6 (+13,200)

k27 – 354,450

When Using Item

• 50% -> 416,950  (+62500)
• 100% -> 479,450  (+125,000)
• 200% -> 604,450  (+250,000)

Breakdown

• Rally Spot Lv26 (125,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 15/20 (+37,500)
• Research – Supreme Coordination 0/25 (+0) * Requires Academy Lv28
• Research – Prestige 0/20 (+0) * Requires Academy Lv31
• Research – Adv Prestige 0/25 (+0) * Requires Academy Lv33
• VIP11 (+10,000)
• Rank – Earl (+20,000)
• Monarch Gear – War Horn Lv6 (+13,200)
• General Equipment – Fearless Dragon Ring: Star5 (+11%)

K28 – 492,200

When Using Item

• 50% -> 564,700  (+72,500)
• 100% -> 637,200  (+145,000)
• 200% -> 782,200  (+290,000)

Breakdown

• Rally Spot Lv28 (145,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 19/20 (+47,500)
• Research – Supreme Coordination 18/25 (+59%)
• Research – Prestige 0/20 (+0) * Requires Academy Lv31
• Research – Adv Prestige 0/25 (+0) * Requires Academy Lv33
• VIP11 (+10,000)
• Rank – Earl (+20,000)
• Monarch Gear – War Horn Lv6 (+13,200)
• General Equipment – Fearless Dragon Ring: Star5 (+11%)

k29 – 621,700

When Using Item

• 50% -> 706,700  (+85,000)
• 100% -> 791,700  (+170,000)
• 200% -> 961,700  (+340,000)

Breakdown

• Rally Spot Lv29 (170,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 20/20 (+50,000)
• Research – Supreme Coordination 22/25 (+82%)
• Research – Prestige 0/20 (+0) * Requires Academy Lv31
• Research – Adv Prestige 0/25 (+0) * Requires Academy Lv33
• VIP11 (+10,000)
• Rank – Earl (+20,000)
• Monarch Gear – War Horn Lv6 (+13,200)
• General Equipment – Fearless Dragon Ring: Star5 (+11%) = 18,700
• Skillbook – March Size Lv4 (+12%) = 20,400

k30 – 772,400

When Using Item

• 50% -> 872,400  (+100,000)
• 100% -> 972,400  (+200,000)
• 200% -> 1,172,400  (+400,000)

Breakdown

• Rally Spot Lv30 (200,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 20/20 (+50,000)
• Research – Supreme Coordination 25/25 (+100%)
• Research – Prestige 0/20 (+0) * Requires Academy Lv31
• Research – Adv Prestige 0/25 (+0) * Requires Academy Lv33
• VIP12 (+10,000)
• Rank – Earl (+20,000)
• Monarch Gear – War Horn Lv7 (+16,400)
• General Equipment – Fearless Dragon Ring: Star5 (+11%) = 22,000
• Skillbook – March Size Lv4 (+12%) = 24,000
• Dragon Celtic Demon Lv6  (+10%) = 20,000

k31 – 873,150

When Using Item

• 50% -> 985,650  (+112,500)
• 100% -> 1,098,150  (+225,000)
• 200% -> 1,323,150  (+450,000)

Breakdown

• Rally Spot Lv31 (225,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 20/20 (+50,000)
• Research – Supreme Coordination 25/25 (+100%)
• Research – Prestige 7/20 (+17,500)
• Research – Adv Prestige 0/25 (+0) * Requires Academy Lv33
• VIP12 (+10,000)
• Rank – Earl (+20,000)
• Monarch Gear – War Horn Lv7 (+16,400)
• General Equipment – Fearless Dragon Ring: Star5 (+11%) = 24,750
• Skillbook – March Size Lv4 (+12%) = 27,000
• Dragon Celtic Demon Lv6  (+10%) = 22,500

k32 – 971,400

When Using Item

• 50% -> 1,096,400  (+125,000)
• 100% -> 1,221,400  (+250,000)
• 200% -> 1,471,400  (+500,000)

Breakdown

• Rally Spot Lv32 (250,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 20/20 (+50,000)
• Research – Supreme Coordination 25/25 (+100%)
• Research – Prestige 13/20 (+32,500)
• Research – Adv Prestige 0/25 (+0) * Requires Academy Lv33
• VIP12 (+10,000)
• Rank – Earl (+20,000)
• Monarch Gear – War Horn Lv7 (+16,400)
• General Equipment – Fearless Dragon Ring: Star5 (+11%) = 27,500
• Skillbook – March Size Lv4 (+12%) = 30,000
• Dragon Celtic Demon Lv6  (+10%) = 25,000

k33 – 1,138,900

When Using Item

• 50% -> 1,281,400  (+142,500)
• 100% -> 1,423,900  (+285,000)
• 200% -> 1,708,900  (+570,000)

Breakdown

• Rally Spot Lv33 (285,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 20/20 (+50,000)
• Research – Supreme Coordination 25/25 (+100%)
• Research – Prestige 17/20 (+42,500)
• Research – Adv Prestige 10/25 (+40,000) 
• VIP13 (+10,000)
• Rank – Duke (+30,000)
• Monarch Gear – War Horn Lv7 (+16,400)
• General Equipment – Fearless Ares Ring: Star5 (+12%) = 30,000
• Skillbook – March Size Lv4 (+12%) = 30,000
• Dragon Celtic Demon Lv6  (+10%) = 25,000

k34 – 1,313,400

When Using Item

• 50% -> 1,470,900  (+157,500)
• 100% -> 1,628,400  (+315,000)
• 200% -> 1,943,400  (+630,000)

Breakdown

• Rally Spot Lv34 (315,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 20/20 (+50,000)
• Research – Supreme Coordination 25/25 (+100%)
• Research – Prestige 20/20 (+50,000)
• Research – Adv Prestige 19/25 (+76,000) 
• VIP13 (+10,000)
• Rank – Duke (+30,000)
• Monarch Gear – War Horn Lv7 (+16,400)
• General Equipment – Plantagenet Ring: Star5 (+13%) = 40,950
• Skillbook – March Size Lv4 (+12%) =  37,800
• Dragon Celtic Demon Lv10  (+15%) = 47,250

K35 – 1,635,000

When Using Item

• 50% -> 1,810,000  (+175,000)
• 100% -> 1,985,000  (+350,000)
• 200% -> 2,335,000  (+700,000)

Breakdown

• Rally Spot Lv35 (350,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 20/20 (+50,000)
• Research – Supreme Coordination 25/25 (+100%)
• Research – Prestige 20/20 (+50,000)
• Research – Adv Prestige 25/25 (+100,000) 
• VIP13 (+10,000)
• Rank – Archduke (+50,000)
• Monarch Gear – War Horn Lv8 (+20,000)
• General Equipment – Plantagenet Ring: Star5 (+13%) = 40,950
• Skillbook – March Size Lv4 (+12%) =  37,800
• Dragon Celtic Demon Lv10  (+15%) = 47,250
• Rally Spot – Special Officer +25% / +50,000
• Rally Spot – Assign Toyotomi Hideyoshi +5%

k36 – 1,769,500

When Using Item

• 50% -> 1,962,000  (+192,500)
• 100% -> 2,154,500  (+385,000)
• 200% -> 2,539,500  (+770,000)

Breakdown

• Rally Spot Lv36 (385,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 20/20 (+50,000)
• Research – Supreme Coordination 25/25 (+100%)
• Research – Prestige 20/20 (+50,000)
• Research – Adv Prestige 25/25 (+100,000) 
• VIP14 (+15,000)
• Rank – Archduke (+50,000)
• Monarch Gear – War Horn Lv8 (+20,000)
• General Equipment – Plantagenet Ring: Star5 (+13%)
• Skillbook – March Size Lv4 (+12%)
• Dragon Celtic Demon Lv10  (+15%)
• Rally Spot – Special Officer +25% / +50,000
• Rally Spot – Assign Toyotomi Hideyoshi +5%

k37 – 1,904,000

When Using Item

• 50% -> 2,114,000  (+210,000)
• 100% -> 2,324,000  (+420,000)
• 200% -> 2,744,000  (+840,000)

Breakdown

• Rally Spot Lv37 (420,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 20/20 (+50,000)
• Research – Supreme Coordination 25/25 (+100%)
• Research – Prestige 20/20 (+50,000)
• Research – Adv Prestige 25/25 (+100,000) 
• VIP15 (+20,000)
• Rank – Archduke (+50,000)
• Monarch Gear – War Horn Lv8 (+20,000)
• General Equipment – Plantagenet Ring: Star5 (+13%)
• Skillbook – March Size Lv4 (+12%)
• Dragon Celtic Demon Lv10  (+15%)
• Rally Spot – Special Officer +25% / +50,000
• Rally Spot – Assign Toyotomi Hideyoshi +5%

k38 – 2,062,000

When Using Item

• 50% -> 2,292,000  (+230,000)
• 100% -> 2,522,000  (+460,000)
• 200% -> 2,982,000  (+920,000)

Breakdown

• Rally Spot Lv38 (460,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 20/20 (+50,000)
• Research – Supreme Coordination 25/25 (+100%)
• Research – Prestige 20/20 (+50,000)
• Research – Adv Prestige 25/25 (+100,000) 
• VIP16 (+30,000)
• Rank – Archduke (+50,000)
• Monarch Gear – War Horn Lv8 (+20,000)
• General Equipment – Plantagenet Ring: Star5 (+13%)
• Skillbook – March Size Lv4 (+12%)
• Dragon Celtic Demon Lv10  (+15%)
• Rally Spot – Special Officer +25% / +50,000
• Rally Spot – Assign Toyotomi Hideyoshi +5%

k39 – 2,219,000

When Using Item

• 50% -> 2,469,000  (+250,000)
• 100% -> 2,719,000  (+500,000)
• 200% -> 3,219,000  (+1,000,000)

Breakdown

• Rally Spot Lv39 (500,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 20/20 (+50,000)
• Research – Supreme Coordination 25/25 (+100%)
• Research – Prestige 20/20 (+50,000)
• Research – Adv Prestige 25/25 (+100,000) 
• VIP17 (+35,000)
• Rank – Archduke (+50,000)
• Monarch Gear – War Horn Lv9 (+24,000)
• General Equipment – Plantagenet Ring: Star5 (+13%)
• Skillbook – March Size Lv4 (+12%)
• Dragon Celtic Demon Lv10  (+15%)
• Rally Spot – Special Officer +25% / +50,000
• Rally Spot – Assign Toyotomi Hideyoshi +5%

k40 – 2,409,000

When Using Item

• 50% -> 2,684,000  (+275,000)
• 100% -> 2,959,000  (+550,000)
• 200% -> 3,509,000  (+1,100,000)

Breakdown

• Rally Spot Lv40 (550,000)
• Research – Coordination 10/10 (+10,000)
• Research – Adv Coordination 20/20 (+100%)
• Research – Super Coordination 20/20 (+50,000)
• Research – Supreme Coordination 25/25 (+100%)
• Research – Prestige 20/20 (+50,000)
• Research – Adv Prestige 25/25 (+100,000) 
• VIP18 (+40,000)
• Rank – Archduke (+50,000)
• Monarch Gear – War Horn Lv9 (+24,000)
• General Equipment – Plantagenet Ring: Star5 (+13%)
• Skillbook – March Size Lv4 (+12%)
• Dragon Celtic Demon Lv10  (+15%)
• Rally Spot – Special Officer +25% / +50,000
• Rally Spot – Assign Toyotomi Hideyoshi +5%

Reference – How to increase march size

The following article explains the details.

10 ways to increase March Size