Either will work. But if you have two or more of the same species they will all lay eggs regularly (~ every 4 weeks) and this will ensure a regular molt as well since they molt right after the eggs hatch.
They also seem to be more "friendly" when there is at least two.
The down side is that they require more nutrition when spawning regularly and can get aggressive in their hunt for food.
I've had 3 for a few years and they get along, although two are paired, the other stays on the opposite side of the tank. Which works out ok because this way there are 2 cleaning stations.