How many fish depends on what size they get, how much filtration you have, how often you do regular water changes and last, but not least, how established the tank is.
In a tank that size, I would recommend dwarf gouramis. They come in a range of bright colours and are lovely to watch. A few bottom dwellers to help keep that level clean and the tank will look lovely.
If you want a mix of different fish, check up on their requirements before you buy. Basically, don't put slow moving fish with long tails in with faster fish with short tails. The faster fish will nip their fins and get all the food first.