At my LFS, at any given time, they have 18-20 neons in a 10 gallon tank. Same for White Clouds Mountain Minnows, etc.
The one side has 30) 10 gallons, three rows of 10 like stairs. This is the smaller freshwater.
Across from that is 12, two rows of 6 stacked, 29 gallon tanks. There's another...