Athens didn't have Helots, who were the Messenians enslaved by the Spartans, but they did have plenty of slaves. The most solid figure we have is that there were about 30 000 adult male citizens at the height of the democracy in the classical period (see Thorley's Athenian Democracy or the recent work by Gotterman). From this, people have estimated that this equated to about 100 000 citizens, and another 100 000-150 000 non-citizens. Many of these non-citizens would have been metics (merchant class or foreigners), but Thorley reckons that there were at least 80 000 slaves at this peak point. His reasoning is beyond me, but it gives us a necessary understanding of scale.

So, about a third of Athens's total population in this period appears to have been the slaves. Discounting non-citizen free population it's nearly 1:1 between slaves and citizens.