Here are some data points:
High school:
Milesplit.com's performance list for high school for the 2016 outdoor season has 3143 boys breaking 2 in the 800m
College:
Milesplit.com's performance list for college for the 2016 outdoor season has 2540 men breaking 2 in the 800m
Directathletics.com's NCAA 2016 All-College Outdoor Track List has 2830 men breaking 2 in the 800m
Directathletics.com's 2016 NAIA Outdoor Qualifying List has 247 NAIA men breaking 2 in the 800m,
so Directathletics has ~3100 college men breaking it in total.
Taking MS's high school data and DA's college data (neither of which are "complete" lists per se, but I would say the majority of HS and college athletes who did it in 2016 outdoor are on these lists), we get 6220 HS and college athletes who pulled it off last outdoor season.
It's extremely tough to say what anyone could theoretically do with 1-5 years of perfect training for the 800m, so I'm only going to make a guess about how many US citizens could pull it off if they had to run it about a week from now (i.e. long enough to get rest, but short enough that they couldn't really train for it). I'd wager 10k or fewer would. Thoughts?