It is really an impossible task to accurately tabulate the number of 5-minute milers in the US. However, we have to assume e every High School in the country has to have at least 5 runners capable of running 5 or under for the mile. Remember there are those on the soccer and football team also capable of these feet that don't run either track or cross-country.
Formulas
Number of High schools in USA X 5
Number of College and Universities in US X 30
Professional athletes capable of running 5 or under = 550
Masters 5 minutes milers = 30,000
My guess there is about 600,000 Americans capable of running a sub five-minute mile.
Back to work - what a pointless thread. Still it is no more relevant than doing a leave blower ad in Indesign. I have been color correcting a picture of a red leave blower for the last 2 hours in Photoshop. It is done so I can think about pointless crap for the rest of the day.