How many female betta fish can you fit in a 5.5 gallon tank?

The general rule is fish keeping is 1" of fish to 1 gal of water. Bettas are 1.5 to 2" long. However, even female bettas will fight until they figure out their pecking order, so give them more room than that and plenty of hiding spaces. Honestly, don't get more than two, and even that may be too many if one of them is a bully.