Adding a couple more fish with tera?

I currently have a 10 gallon tropical tank with 2 otto, 5 black neon tetra and 3 glow light tetra. They seem to be happy and stick together in a school. Just wondering if a couple more tetra or otto would be happy to join them - ie is there enough room?Adding a couple more fish with tera?
The general rule of thumb is one gallon of water per one inch of adult fish. So if you had 10 one inch fish, you would need at least 10 gallons of water. So basically, it depends how big your fish are and how big they will get because you don't want to crowd your tank. That whole thing with fish only grow to the size their tank lets them is a myth. Good luck!!