There is no "rule" there are general guidelines for a suggested maximum. A common guideline you will see is 1"/25mm of fish per gallon. That assumes a normal shaped tank however, tall tanks and especially hex tanks have a smaller surface area so should not have as many fish in them.
Neons can grow to more then 1"/25mm but rarely do. Without further knowledge, I would say that 10-12 Neons is a better estimate.