Interestingly, security clearances and their procedures are under the control of the President and can be changed by executive order.
More than a million people had ‘Top Secret’ clearance as of a few years ago.
Background:
https://fas.org/sgp/crs/secrecy/R43216.pdf
With that said, I can’t believe Hillary Clinton did not know better than to send documents through a private server (unsecured) which were classified. Even a low level employee like myself knew better.