Neons are one of the species that are lighter on the bioload of the tank, hence it can probably handle more than 1 inch per gallon. (I assume you have a 2ft tank). Are you planning for other fishes as well?
The tank is 2ft long. I have currrently housed 2 African Clawed Frog's with 5 neon tetras. The frogs have reached full maturity and after having the species together for over 2 weeks I have had only 1 neon casualty.
Sorry I really have no idea how much space/bioload do frogs take. If you were keeping neons alone, I'd say you can easily keep 15, but with frogs, I have no idea...