Russia only has about 200,000 soldiers
Ground forces have 230K troops alone but you should add Airborne troops,Marines and National guard to it. Number will double.
Also needles to say that if Russia feels threatened, they will call in millions of cannon fodders. Za rodzinu,Za Putinu.
Actually, Finnish membership in Nato would probably be the end of Russia's ability to actually defend itself from a Nato conventional attack.
Russian army is tailor made for exactly such kind of attack. (this is why they suck in conventional conflicts). Front line stretched over thousands of KMs, where Russian artillery and tanks will feel like fish in a water.
Also don't forget that,just like in good old days, Russia has huuuge strategic depth, they can easily continue fighting even if enemy somehow reaches A-A line.