Hi 101! What you need to realise is that it is not a mere question of cms- you need to take the shape of the tank and the needs of individual species of fish into account.
Assuming that your tank is rectangular, it should theoretically be able to house 8 inches of fish= 20 cms. However, and this is the important bit, because it is a small tank, you will not be able to add any fish larger than 2 inches or so, no fast swimmers (like danios), no territorial fish (unless kept on its own- like a male betta), and schooling fish will need to be calculated very carefully, to get the proper numbers.
Once the tank is fully cycled (read up on pinned topic if you have not already done so), it should have room for something like a small school of glowlight tetras, maybe even together with 3 pygmy corys. Or a group of endlers. Or a betta and some small corys. Or some African dwarf frogs. Or (but only just) for some platies. There's quite a few different options, but make sure you don't add anything until you have thoroughly researched it- less space means less margin for mistakes.
At all costs, avoid anything labelled a pleco, anything labelled a shark, also mollies, swordtails, danios, and Chinese Algae Eaters.