Modern scholars estimate that Athens began the war (in 431 BC) with a population of about 60,000 adult male citizens. The total population (including women, resident non-citizens and slaves) is impossible to know, but it would certainly be many times that number.

By the time of the coup of the Four Hundred, twenty years into the war, the population had been decimated by the plague and disasters like the Sicilian Expedition. It is generally held that the number of adult male citizens would have dropped below 25,000.

The Four Hundred were an oligarchy of the richest citizens, a group smaller even than the Athenian cavalry, which was manned exclusively by the rich and numbered 1,000 (although losses would have cut them down to size). They tried to base their rule on an assembly of 3,000 citizens who were selected based on how much property they owned. The regime of the Four Hundred was soon overthrown and replaced by the more moderate oligarchy of the Five Thousand, who by this time must have included just about everyone on the draft lists as cavalry or hoplites. Soon after that, the democracy was restored.

Edit: had to check M.H. Hansen to make sure that the figures of 60k and 25k did indeed include only adult male citizens (i.e. men over 18).