If the rabbim in a minyan during chazaras ha-shatz don’t say amen to any of the 19 brachas, does the bal tefillah trangress a bracha-levatalah?
There is a dispute among authorities as to how many people need to answer the chazzan’s berachos: some say nine have to answer, and any less than this would constitute a berachah levatalah. Others write that eight are sufficient, and others maintain that six people answering are enough.
Mishnah Berurah writes that when one is unsure about the presence of nine people answering, one should preferably made a condition, that if nine people will not be answering, the tefillah should be a nedavah. However, other poskim have written that one need not be particular for making this condition.
Sources: See Shulchan Aruch 124:9 who writes that nine need to answer; yet see Magen Avraham 55:8 who writes that it is customary to rely on there being eight, and brings opinions that rely even on six, and see Mishnah Berurah 124:19. See also Mishnah Berurah 55:32. Ishei Yisrael 15:11 cites that some make a distinction between Kaddish and Tefillah, but this distinction does not emerge from Biur Halachah (55:6).
Although Mishnah Berurah (124) recommends that one make a condition for a nedavah, see Halichos Shlomo (9:96) where Rav Shlomo Zalman is cited as stating that he used to make the condition, but later ceased to do so, preferring to set his fate together with the rest of Klal Yisrael — who don’t make the condition. See also there (24:4) in the name of Rav Chaim Kanievsky.