There is no difference in Vietnamese between how many and how much, both are bao nhiêu. As it works as a kind of quantifier (showing/asking how much or how many something is), it is followed by the classifier in Vietnamese, and then by the noun. So I don't think it is possible to say Có bao nhiêu những con chó, because then you'd have two quantifiers; it's like saying how many several pieces of cake in English.
My understanding from the notes is that if you want to clarify price, you can use the word "giá" which means price.
"Noun + giá + bao nhiêu? (giá means “price”). (E.g: Cái mũ này giá bao nhiêu? (How much is this hat?))"
Also in this exercise, I think the word "Có" specifies having, or there being, so to me, "how many" seems to be indicated more than "how much".