I have had 6 Mini Rex litters in the year I've been raising them. The litters have been:
3,4,5,6,6,7.
It really can vary depending on the doe. I know a breeder, who said that one of her does would fluctuate her litter size. I think she had 3, and then 9 and then 2 and then15! So yeah. It kinda depends on the doe (ithink 15 is pretty uncommon though lol)