Wycherley cites A.W. Gomme, Population of Athens, for that figure. Gomme's book was published in 1933, so that answer may be subject to serious revision in the meantime. Also, Wycherley specifies Periclean Athens — the effects of the Peloponnesian War may have significantly altered the population of Athens, so much depends on when in Socrates' lifetime you're inquiring about.
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