"Bu grubun kaç üyesi var?" Translation: How many members does this group have?
Wht isn't "This group has how many members?" Correct?
The members (quantity) is being asked about & not the group. The group (subject) exists & the size of it is not known.
üyesi - members (accusative) case "the" definite members a quantifiable direct object or group of objects.
üye - member (s) buffer consonant & -i front vowel (dotted i) "the" member - üyesi.
The members are the objects & (kaç) being an adjective after the (noun) - group is asking the total amount or quantity of those members. Var is the verb for "are" - "how many are."
Your word order is wrong. Sorry for not being able to explain this further.