One tops. Don't be cruel with them in terms of space. Remember, the hight doesn't matter as much as the foot print. Think of them like fish; more than one (unless maybe you had a pair) would not be happy in that space
a 20 long is perfect for a pair or a trio anymore you will need a larger foot pring they grow to be arround 10 inches so keep that in mind they are also nocturnal so a red heat lamp or ceramic heat emiter is needed if you need anymore info let me know i used to breed them for years