Thats a bit of an impossible question to answer really... it could be 2 if you wanted a pair of breeding Cichlids or it could be 50 if you wanted just a school of Micro Rasbora?
If we can ask some more quick questions, that would help us help you
What is the capacity of the tank?
What filtration do you have?
What fish do you currently have?
How long has the tank been set up?
What is your ph?
Is your water soft or hard?
Do you know your gh and/or kh?
What sort of fish do you like?
What sort of substrate do you have - gravel or sand?
Do you have plants in the tank?