How many cardinal tetras fit in a 20 gallon tank?

The tank is heavily planted. I'm planning on getting a few otos as well. How many otos and cardinal tetras should I get? There are no fish atm. Also, when adding the cardinal tetras/otos, how many of them should I add at a time? And how long should I wait before adding more? I want like 10-15 cardinal tetras. I have 1 assassin snail and a red cherry shrimp. Planning on getting a few more shrimps. Also, is 20 gal too small for discus fish? I read that 10-15 gal is minimum for a "small" discus fish. What are "small" ones. Do they grow into bigger ones?