Finding homes can always be an issue for anyone. I would think that the better solution is not to breed more than you can afford to live with for as long as it takes rather than giving a specific number of times to breed each cat. If I have a litter of 5 kittens, it's fairly easy to keep each one until the perfect homes come along, but if I have 3 litters that each have 5 kittens, and they aren't all adopted by 6 months of age - and then I have more litters coming, then I would probably feel a lot more pressed to get them out of the house ASAP. That would also lead to some irresponsible placements I'm sure.
I'm not accusing Savannah people of doing that, but I've seen a few local dog breeders that work that way. Each dog has to produce a certain number of pups before they can retire - and the quicker the better to them. I don't know a lot about how Savannah, or any other new breed people go about things, so there might be some logical reasoning to it that I don't comprehend.