I do not know an exact statistic. Mark Twain said that there are three kinds of lies: Lies, Damned Lies, and statistics.
Becoming a Mason is a fairly simple procedure, but there is a procedure, nevertheless. The basic requirements are:
Male
Mature of Age (18 some states, 21 most states)
Belief in God
Good moral character.
Some states impose a residency requirement, Georgia three years, California one year. Some states exempt active duty military from the residency requirement.
Some states require the petitioner to have two masons sign his application(petition) form. Some states require that the petitioner list three individuals who can vouch for his character (they need not be masons).
Most states require that the petitioner have no felony record. Some states evaluate the candidate on a case-by-case basis.
Because the requirements are broadcast out in the open, and no effort is made to keep them secret, a man can easily know, if he meets the requirements for Freemasonry. If he is clearly not qualified, then the lodge will dismiss his application immediately, and no record is kept.
I wish I could be of more help, but a question like this really has no definite answer.