I've never done well with neons in small tanks. They like to be not only in schools, but in big schools, preferring to be in groups of 10-20 or more, up to 100's.
10 neons would be awfully crowded in 10 gallons.
To give a direct answer to your question, I wouldn't say more than 5, however, I wouldn't say "have neons in 10 gallons"....
They will stick together like glue in one area of the tank, about an inch or two apart, if they're anything like mine were... They have a better visual impact that way than scattered anyhow.
If you have a school and they get reduced to two or three fish (like mine did....terrible tankmate issues...), they will scatter and will be unhappy....
Someone else's opinion might differ, it's been a long time since I had any tetra breeds....I moved over to cichlids and cats long ago...