I'd say somewhere in the neighbourhood of 80, posiibly 100 if you have it heavily planted (would be nicer for the fish).
If you are doing it to see a nice school, it won't happen. Fish need a threat to school (it's a defence mechanizm).
try adding 5-6 Diamond tetras, then 15 Neons and see if they start schooling (the size of a diamond is big enough to be a threat, and small enough that you aren't stocking feeder neons like I was with the congos (lost 21 neons in 1 week, 0 bodies and none were injured, the congo's doubled in size).
Also, Cardinals and Rummynose make nice schools, you could set up a school of neon's or cardinals and a school of rummynose.