Up to ch.12 I always thought there were only four of them: Charles, Henrietta, Louisa and poor Richard. But in ch.13 we read of the old nurse who had stayed till long-petted master Harry had left for school and later on in ch.14 we read there are even more:

The Musgroves came back to receive their happy boys and girls from school.

Supposing little Harry is one of them there are at least two more girls and another boy. If I counted correctly there must have been at least eight children! That throws another light on the Musgroves IMO. They were not only frank in their relation to one another (as we already discussed) but seemed to be also a vibrant, loud and happy family with an open house. Perhaps I should reread Mrs. Musgroves comment on Maryīs parenting - she seems to be the one with experience rather...

Itīs also interesting to read a little further on in ch.14:
Immediately surrounding Mrs. Musgrove were the little Harvilles, whom she was sedulously guarding from the tyranny of the two children from the Cottage.