Here's a fun 2011 Sports Illustrated article by Pablo Torre on retired NBA seven-footers:
Anactual accounting of 7-footers, domestic or global, does not exist in any reliable form. National surveys by the Center for Disease Control list no head count or percentile at that height. (Only 5% of adult American males are 6'3" or taller.) "In terms of the growth spectrum, 7 feet is simply extreme," explains endocrinologist Shlomo Melmed, dean of the medical faculty at L.A.'s Cedars-Sinai Medical Center. The term 7-footer is itself a kind of outer limit, a far-off threshold beyond which precise measurement seems superfluous. A 6'4" guard isn't a 6-footer, after all. The curve shaped by the CDC's available statistics, however, does allow one to estimate the number of American men between the ages of 20 and 40 who are 7 feet or taller: fewer than 70 in all. Which indicates, by further extrapolation, that while the probability of, say, an American between 6'6" and 6'8" being an NBA player today stands at a mere 0.07%, it's a staggering 17% for someone 7 feet or taller.
There are a lot of issues with this, but it's not a bad first cut at the question.
The article focuses upon iSteve favorite Mark Eaton. I learned an important lesson from him when we were at UCLA in 1981.