Thing is, I think they are all incredibly fertile cause Tsukasa was truly born to become a mother. However, I think it depends on the AU on how many kids would Rui and Tsukasa have. Because some AUs are set in the past, for example, Gunhina or Kuroshiro would have more kids since Tsukasa is kinda raised to become a good wife. We should also take in considerations AUs like Octopus/King or Wolf/Bunny Dantoru, since animals characteristics can affect, Octopus!Rui probably gets King!Tsukasa full of eggs but how many of them survive or bnnuys usually having 4-8 baby rabbits... much to take into considerations really. I like them having like 4 pups max though lol