Via Dan Diamond:

An actual 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.)…

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.