15 cogs, all occupied, all alive and relatively happy. I'm pretty sure it says 15 but bugged out at some point.
Recently one of my dups died from suffocation, I reloaded from autosave and saved her.
Found reason - rockets.
1. Assign dup to rocket.
2. Wait till he jumps in
3. Population counter drops by 1.
Not sure if that intended behavior or not.