you CAN have multiple litters of kits...if you're willing to cheat on your mate secretly >:D I actually do it a lot because it's fun to get all the possible mates. The way to do it is to start as a girl. Next, become a warrior and immediately become mates with Breezesong. Afterward, go up to where Nightwhisper is and become mates with her. Then, become mates with Dustfeather (it might take a few tries), then while he's in the nursery with kits, become mates with Duskfall. They don't care if you try for kits near each other, they only care if you ask them to be mates with you. Then, once that's done, go to the rouge and change your gender to male. Have kits with Nightwhisper, Cottonwillow, and Fallenfeather in that order. By this point, you can become leader and then get the adopted kits with Breezesong.
I know it's evil but this way you can have six litters of kits plus the two adopted ones. I AM a Slytherin :)))))