Does the chazzan have to wait for 10 men to say kaddish after shmona esrei of ma’ariv?
According to halacha the chazzan may say kaddish or borchu, if six people are listening. The reason is because kaddish and borchu are a “davar shebikidusha”, and they need ten people there together, however if most of them are answering, and there are a few that are listening, even if they can’t answer because they are in middle of shemona esrei, it may still be said. There is however an opinion that it shouldn’t be more than one person of the ten should be still davening (eight people answering to the kaddish etc).
This is different that saying chazras hashatz, because since the chazzan was already yotza shemona esrei, and he is only saying it for the people to hear it, then we specifically need nine people answering.
O:CH 55-6, M:B ibid 32, see S:A Horav ibid 7, and Kitzur S:A 15-7, Aruch Hashulchan 57-13, that don’t bring this stringent opinion. Eishei Yisroel 15-3. However the Aruch Hashulchan says that if they don’t have to wait long it is better to wait so he will also be able to answer kaddish.