How many fish can I put in my aquarium?

I havent bought anything for this setup yet, i am just mapping out what is achievable. So i want a 15 gallon to 20 gallon planted aquarium. Easy plants, java moss and fern, Elodea and the like, low maintenance but heavily panted. What I'm hoping for is some cherry and ghost shrimp, 2 cory catfish, a small school of tetras and some mickey mouse fish (aka platy's). And a betta, is this over stocked? how many of the tetra's and platy's can I keep? I already have my betta in a tank and he does just fine with other fish, honestly im more worried about the fish hurting him.