Hello and welcome to the community! Usually, in a well-maintained aquarium, you should be able to have one inch of fish per gallon. One gallon is a little bit more than 3.5 liters so the maximum number of fish in a 72L aquarium would be: 19. I would recommend starting with half that many until your tank is well established. The easiest fish I have ever raised were Molies. They give live birth and seem to do well in cool to warm water. I'm sure lots of others can offer better advice about the kind of fish as my experience is limited with them. I usually just grow water plants in my aquarium and add in some fish for fun.

cheers,
BD