When the chazan repeats the Amidah, each and every congregant must be silent, have kavanah to hear the chazan’s berachot, and answer Amen to them all. We have already mentioned the opinion (in the laws of the minyan, 2:7) that the chazan is prohibited from starting to recite the Amidah repetition until there are nine men who will respond Amen to his berachot. Others say that even if a few members of the minyan are still praying the Amidah, although they cannot answer Amen, they are counted as part of the minyan.
L’chatchilah, it is customary for the chazan to be stringent and not start reciting the Amidah repetition until nine men are available to answer Amen after him. In extenuating circumstances, when there are people who are in a rush to conclude the prayer service, and it seems that the person extending his Amidah is not about to finish, it is permissible to rely on the opinion of most poskim and start Chazarat HaShatz when only eight people have concluded the Amidah. When this is impossible because there are a number of people prolonging their Amidah, if the situation is extremely mitigating, it is permissible to be lenient and begin the Amidah repetition when only five have finished praying, for together with the chazan they comprise the majority of a minyan. In order to avoid uncertainty, if it is necessary to begin the Amidah repetition when there are not yet nine people able to answer, the chazan makes a stipulation in his heart before praying. He stipulates that if the law follows those who maintain that there must be nine who answer, his prayer is voluntary (tefillat nedavah). Since a person is permitted to pray voluntarily, according to all opinions his berachot will not be in vain.
Similarly, one who is leading the prayer service in a place in which many people normally chatter, so much so that there is doubt as to whether will be nine men there to respond Amen to his berachot, he makes a stipulation in his heart before the repetition, that if the law follows those who maintain that there must be nine who answer, if there are not nine, his prayer is voluntary.
In order not to enter into such uncertainty, every person who hears Chazarat HaShatz must presume that without him there are not nine men; he must concentrate on the chazan’s berachot and respond Amen to them (Shulchan Aruch 124:4).