They start getting territorial as adults, so if in a tank together, provide plenty of hiding spots (fake plants, pvc, clay pots, etc) to break up the line of sight from each other. They get vulnerable too when molting. The more hiding spots, the more that can be kept in a tank. Tank size is subjective. I've kept 5 adults in a tenner before without problems because of "clutter." Having said that, I currently have a pet Marm that I keep in a cheap tenner bare tank, by herself.
Reason for keeping influences choice of tank size. If kept as a pet, get a tank or container as large as you wish so you can enjoy them. If a feeder production factory, I've seen fish keepers keep one each in storage containers, and 5g buckets. All depends on your intentions.