I would have no more than 6 in a small group. When I taught third grade, 4 was the ideal number for me. That way we could have a conversation without it being too chaotic and I could have them turn and talk to each other and listen in.
My tentative planning for groups this year... Last year I had 30 kids and I won't be surprised to hit that again. Starting with 25. I plan on doing 6 groups. Some will be small and some will be larger with no more than 6. I like them small enough I can pull some of the kids into the conversation better and have more access to them.
I've always done 4-6 in a group as well. I was just confirming and wondering if she suggested a different number. With class sizes (generally) being what they are I think it's unrealistic to expect anything smaller.