Is a 20 gallon tank big enough for a beared dragon?

Technically, you could but they probably won't mate. At least a 40 gallon cage is required for one adult bearded dragon, and larger is recommended for two. If they are being mated, they need a lot of space and they have to be able to access some kind of rock or log that is higher up than the others, so the male can feel dominant when he climbs on top. The substrate in the terrarium needs to be something that can be dug in to make a proper burrow that doesn't collapse, but make sure it's digestible otherwise they could get impacted and die. Once the eggs come out, they need to be stored in an incubator, and then the babies need another terrarium of their own, after they hatch, so the parents don't eat them.