How many fish can you put in a 14l tank?

Not very many. If you don't have a filter and air pump, maybe two or three *small* fish. Like, an inch or 3 centimeters each. Having some plants would be good, because the plants use the fish 'pee' to grow, and that helps keep the water clean. But even so, with a 14-liter tank, you should change SOME of the water once or twice a week (remembering to use a de-clorinator before putting in the fresh water).