A school of 6-7 of the medium tetras would be close to max in your tank. I have found w/ the tetras I have they school a lot less then my danios. They kind of hover in the tank unless something stimulates them. I only have 3 neons because 7 of them died during QT. I was drained treating my betta for fin rot in another tank during the same time so I have not replaced the neons that died yet. Neons are very delicate in my opinion. Neons can get tetra neon disease which can not be cured. Nothing new though as far as disease goes because even the disease that can be cured supposedly is hard to cure because fish seem to be immune to antibiotics when they come from the fish store. Neons like a PH of about 6.5 and soft water. They can grow to about 1 to 1 1/2 inch. Each fish can vary on the type of water they thrive well in so it would be best to match the water you have to what the fish you would like thrive best in.
My dainos schooled in a pair of 2! Now I have eight. They are wild.
Is your tank cycled? Do you want more then one type of fish? If your water is right perhaps a school of 5 of the small tetras like the neons (my neons poop very little compared to the bigger tetras) and maybe 5 of another small tetras you would like. That would be a bit over stocked but if you do weekly WC's it should be fine.
This might help you decide: http://www.tropicalfishandaquariums....tras/index.asp
55g - 2 green spotted puffers in brackish conditions
20g- 6 leopard danio,2 glofish danios,4 whiteskirt tetras,shrimp & neritas
10g- ramshorn & pond snails,3 neon tetras, cherry shrimp
10g- QT tank-empty
2.5- QT tank- nerita,amano,anacharis