citizens

Only your computer's ability to handle them all.   

luceo non uro

only 10 will fit in each slot on the Govern screen, so you can only have 30% (60% with leaders) total global research (for example) from citizens.  Most of the time you can transfer them to planets and/or promote, but it is really annoying if there is no E. Pollen around, and you already have 10 administrators.

But there is no hard limit on the total number of citizens in general.

After the first 10 slots you could add scientist and engineers etc to planets.