For maariv is it ok to have 8 people answering or do you always need 9 besides BT (and the baal tefillah is never counted)?
For maariv it is alright for the chazan to start kaddish after shemona esrei if there are 6 people that already finished and will answer kaddish, it is however preferable that there be 9 people that already finished (including the baal tefila). This is specifically for maariv when it is kaddish being recited, however for shacharis and mincha, where the shemona esrei is being repeated, we will need 9 people that will actually answer amen in order to start. (In extenuating circumstances it is permitted if 8 people finished davening and one is still davening)
M:B 55-32, O:CH 124-4.