The figure I remember from many (many!) years ago is 20 percent. Who knows which direction it has changed since, or how accurate it was even then. But as I recall, this related Ph.D.s awarded in mathematics to the steady-state number of faculty in mathematics departments at research institutions. (Both in the US?) Of those getting a Ph.D. in mathematics, only 20 percent would end up in a faculty position at a research institution. The other 80 percent would end up (perhaps after a temporary stint or two) elsewhere ... 4-year or 2-year colleges, Wall Street, National Security Agency, other industry, etc., or even cab driver or unibomber.