To comprise a minyan, it is not necessary for all ten men to be able to participate in saying the matters of sanctity. Even when a few of those participating cannot respond to the chazan, they still complete the minyan. For example, if a few of them have not yet finished reciting the Amidah of Ma’ariv, even though they cannot respond to Kaddish and Barchu, the Shechinah dwells among them because there are ten Jews present. As a result, they are permitted to recite matters of sanctity. Likewise, a deaf person who does not hear the chazan, or a mute person who cannot respond, can each still complete a minyan. There must be at least five people present who answer the chazan, since together with the chazan, they make up a majority.
In the opinion of some prominent Acharonim, the ruling that even one who cannot answer may be counted as part of the minyan refers to Kaddish and other matters of sanctity. However, regarding Chazarat HaShatz, it is necessary to have nine people who will actually answer Amen to the chazan, for without nine, his berachot will be in vain (Shulchan Aruch HaRav 55:7; Ben Ish Chai, Vayechi 6). Nonetheless, according to many poskim, although l’chatchilah it is necessary that nine people answer Amen to Chazarat HaShatz, in principle, even those who do not answer Amen count, and the chazan may start Chazarat HaShatz even before all nine have finished praying. Similarly, it is permitted to repeat the Amidah in a minyan in which people regularly engage in idle chatter during Chazarat HaShatz, even if there might not be nine people answering Amen to the berachot, since b’dieved even those who do not answer still count as part of the minyan (Magen Avraham; Eliyah Rabbah).
In order to avoid uncertainty, in a place where many people chatter and there is concern that there will not be nine people answering Amen to the chazan when he repeats the Amidah, the chazan should stipulate in his heart that if the ruling is according to those who maintain that nine people are needed to answer, then his Chazarat HaShatz will be considered a voluntary prayer (tefillat nedavah). Since a person is permitted to pray a voluntary prayer, according to all opinions his berachot will not be in vain (Mishnah Berurah 124:19).
.At first glance it seems that there is a contradiction in what the Shulchan Aruch writes, for in section 55:6-8, he rules that a person who is praying, a person who is sleeping, and a deaf person are also counted in a minyan, despite the fact that they cannot answer Amen after the chazan. Whereas in section 124:4, he writes based on the Rosh, “And if there are not nine people who have kavanah during the chazan’s recital of berachot, his berachot are close to being said in vain.” In the opinion of most poskim, among them Magen Avraham, Derishah, and others, even those who cannot respond to the chazan are counted as part of the minyan. Therefore the Shulchan Aruch emphasizes “are close to being,” meaning that in actuality his berachot are not said in vain. That is what is implied from the Beit Yosef in section 55 who brings the words of the Maharil and the Ram, maintaining that a person who is sleeping and someone who is praying can be counted for Chazarat HaShatz. (There disagreement is about how many people that do not actually answer can be counted. The most lenient opinion allows up to four. Regarding one who is sleeping, we may not be lenient with more than one, Mishnah Berurah 55:32.)In contrast to them, the Taz 55:4 and 124:2 is inclined to say that only those who answer are counted in the minyan. The Graz 55:7 and Ben Ish Chai, Vayechi 6, differentiate between all the matters of sanctity about which the Shulchan Aruch writes in section 55 that even one who does not answer counts, and Chazarat HaShatz about which the Shulchan Aruch writes in section 124 that it is necessary to have nine people answer.
In extenuating circumstances, when there are people who extend their prayers and it is difficult to wait for them to finish, it is possible to heed the advice of the Mishnah Berurah 124:19 – that the chazan stipulates that if the ruling is according to those who maintain that it is necessary for nine people to answer, then his prayer is considered a voluntary prayer (tefillat nedavah). In the Musaf prayer and Shabbat and festival prayers, the chazan cannot make this stipulation because on Shabbat and festivals a voluntary prayer may not be recited. Instead, when he is concerned that nine people will not respond to him, either he should rely on the majority of poskim who say there is no need to have nine people answer or he should not pray the silent Amidah with them and this way he can recite the Amidah repetition without doubt, for he himself is fulfilling his obligation through this prayer.
On weekdays, when the minyan is smaller, we can rely on the advice given further in this book (19:5) that the minyan may pray the abridged Chazarat HaShatz and the chazan recites the first three berachot out loud in order to say Kedushah.
According to almost all opinions, a person who prays word for word with the chazan is counted as part of the minyan even though he does not answer Amen.