Two kittens. The clues tell us that grandma Gemma bought more than one kitten (kittens). More than two would have resulted in too many legs over-all, therefore she must have bought six chickens and a legless pet, a total of 9 heads and 20 legs.
The question asked for the number of kittens only. It doesn't matter what Charles got but if you really want to know, it was a goldfish.