"Do you have some?"
Haben requires accusative. If the subject is not explicit singular masculin is implied. So, we have acc. singular masculin from welche which is welchen. It would translate as Do you have some of that?
I agree with zimator and saladuo, "haben sie manche" should be marked as correct. At this stage Welchen has not been introduced to any lessons. Welche was introduced and translated as "which (plural).
I am totally agree with your point. "Welche" means Which or What not some. This is weird translation.
Manche would make more sense as we haven't yet learned the other constructions.