"How many members does this group have?"
Translation:Bu grubun kaç üyesi var?
I am not Ruth, I don't know if she still learns with Duolingo as her comment is 4 years old. So I am trying to answer you. I think that you are mixing 'soru = question' with 'sorun = problem' So, 'I had the same question' would be '(benım) aynı sorum vardı'' veya 'Aynı sorum bende'
Why not "var mI"?
mı is for yes–no questions*, not for "WH" questions that have a question word such as "what, why, how, how many, where, who, ...".
Bu grubun kaç üyesi var? would be a bit like asking "Does this group have how many members?" -- using the verb-first ("does ...?") style of yes–no question in combination with a "WH" word ("how many?").
* Well, and also for "pick one" questions such as "Do you want coffee or tea?".