The best is if the Minyan could wait patiently for everyone to finish. If, however that is not tenable, then you should wait for nine who can answer the Chazzan (the Chazan being the tenth.) If that also can’t work, for example some men might just leave anyway, or it will cause fights then once can be lenient and have five men (who listen and answer) plus the Chazzan.
Explanation:
The Tur (124:4)quotes the Rosh about Chazarat HaShatz: And the Chazan prays to fulfill praying he Shemoneh Esrei for those who don’t know the Tefilla, and the congregation must be quiet and focus on the blessings of the Chazzan and answer Amen. If there are not nine in the Bet Knesset listening to the blessing of the Shliach Tzibbur then his brachot seems to me to be fo naught since the Brachot of the Shliach Tzibur were set up to be said with then men and if they not focusing and listening to his Brachot then he is saying Brachot or no reason.”
The Rosh implies there needs to be nine men who finished their personal Tefilla to be able to listen and answer all the Brachot of the Sha”tz.
On the opposite side, the Hagahot Maimaniyot in the name of the Mahar”am as quoted by the Bet Yosef (55:6)say “If one man is still praying by himself and he can’t answer with the rest of the minyan and there are only eight men who can answer, this late ‘prayer’ is added to their number to make ten, and it says, “And I will become holy,” the Shechina comes. Meaning any ten men is good enough
It seems the Maharam and the Rosh argue because he says that if only eight are answering to the Chazzan you can start Chazarat HaShatz and the Rosh writes specifically that you need nine. The Shulchan Aruch rules like both opinions.
In 55:6 The Shulchan Aruch rules like the Mahram tht if one is still praying, he makes the minyan and in Siman 154:4 he rules like the Rosh that you need ten. There numerous answers to this contradiction, we will quote the ones which are relevant to Halacha.
The Drisha in Siman 124:1 explains: the actual Halacha is like Siman 55 that we only need eight answering. In Siman 124 he quotes the Rosh verbatim” It seems to me that the Bracha of the Shliach Tzibur is for naught,” he only writes “It seems to me” but it is not actually a Bracha Levatala (a wasted Bracha). Since we only ideally need nine men answering but is enough with eight.
The Magen Avraham (55:8) explains: Even though the Rosh writes that we need nine men listening to the Brachot of the Shatz, are custom is not like that, since we see that even one who is talking and to paying attention can be counted towards the ten. The Elya Rabbah and Derech HaChayim agree.
The Shulchan HaAruch HaRav (55:7) explains: in Siman 55 The Shulchan Aruch was discussing Kaddish and Barchu and even Kedushah but for Chazarat HaShatz one needs nine.
What comes out is that according to the Drisha you need eight, and according to the Shulchan Aruch Harav you need nine. The Magen Avraham quotes the opinion of the Reblochon says, “eight does mean eight but even if three or four are still praying as long as there is a majority who are answering.”
The Ralbach is lenient even if you have only five people who finished plus the Chazzan. The Magen Avraham doesn’t agree, but the Derech Hachayim and Elya Rabbah agree to the Ralbach and only require a majority. To listen and focus and answer for Chazarat HaShatz. The Mishna Berura is lenient like this opinion as well that you only need a majority.
Therefore, in a case of need one can definitely rely on the Mishna Berura and start with only a majority (5+1). If it is not too difficult to wait, then you can fulfil the Shulchan Aruch Harav ( and Rosh) and have nine to answer. It is even praiseworthy wait for everyone if possible, to give everyone a chance to answer, as it says on Shulchan Aruch Siman 55:7.
