... there are about a half a million of them on supply.
Don't mind if I do quote myself...
I think I misspoke or at least wasn't specific. There aren't half a million slaves on hand at AL, there are half a million tonnes of slaves on that station alone. I think that means there are roughly 8 million slaves "on the shelf" there (at most). And then there are slaves at many of the other stations in the Sol system.
I don't think this fits how the seat of the Federation (yeah, I know the seat is on Mars, but same system) is portrayed. Nevermind the whole "We sell things we prohibit" part.