If you're looking for peaceful community fish, then I would definitely recommend guppies, endlers, platys, rainbowfish, rasboras, corydoras, gouramis, etc.
just make sure if you get a schooling species that you keep them in groups of AT LEAST five.
There isn't really a set rule as far as how many different species you can keep in the same tank, as long as their water condition needs are the same and they have enough space.
Tetras can be great community fish too, but I didn't list them previously because they tend to prefer more acidic water whereas the other species prefer their water to be more alkaline. I wouldn't recommend trying to keep tetras with some of the aforementioned species because it is impossible to create an environment that will be hospitable to all of them. They will become stressed. Not to mention certain species of tetras can be fin-nippers.
But, if you like the idea of having tetras, that opens up a whole other realm of possibilities.
My point is, really research what parameters are ideal for the species you are interested in (saltwater/brackish/freshwater, pH, temperature, etc.) and then find tank mates with similar needs. It will be much easier for you in the long run and your fish will live longer and will be happier than if you try to force them to adjust to different conditions than they are used to.
P.S. I can't recommend aqadvisor.com enough!
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