I do believe many keepers keep Xenopus species together as long as they are similar size. In a 20g I'd say 3 would work nice, large females are very active if well kept and will need the space and I hear they really like company. I know my own personal frog will never live with a tankmate, she is just too aggressive. I guess if you get them young and have them grow up together they will be fine.
BTW, mine jumps out of the water 3 inches to try to eat my fingers, attacks the fish net, attacks the water current etc. Eats literally anything.