There is no ideal number. The factors that will decide how many fish you can keep include territorial space of species you choose, species compatibility and your filtration's ability to control nitrate and phosphate levels.
Decide what species you are interested in keeping and research their territorial space requirements. Research their compatibility with each other.
Introduce the least aggressive first and wait for a month or more between each addition. During this time test your nitrates and phosphates to determine if your system can cope with more additions.