especially in our culture in which by the time a woman reaches the age of 20 she'll likely have had at least 5 previous sex partners.
You are being overly generous to the younger millennial set.
Let's assume that a modern woman becomes sexually active at 15. If she sleeps with 4 guys a year, her notch count is already at 20.
She would only be halfway through college. In my college days, the average girl was sleeping with a different guy way more often than once every 3 months.
You can easily double or triple her notch count if you count only oral sex. This is what is leading to a head and neck cancer epidemic. HPV rates were exploding before the vaccine.