Q. In order to fulfill the mitzvah of tefilla b’tzibbur, is it necessary to have 10 men davening S”E (Shmone Ezreh), or is it enough to have six of them davening S”E while the other four are just standing around, or learning Torah, or reciting other parts of davening?

If it is necessary to have 10 men davening, then in a case where one finds himself in a place where only six (or seven or eight or nine) of them will be davening, what should he do? May he also daven with that minyan?

A. Tefila betzibur (public prayer) requires a minyan comprised of ten men. Poskim disagree and some rule that you have to have a complete minyan of ten individuals who did not daven yet for the prayer to be considered as tefila betzibur (Chaye Adam 19, quoted in Mishna Berura 90: 28, Igros Moshe O.H. 28 & 30, Yad Eliahu ps. 7, Teshuvos Vehanhogos 1: 120 quoting Brisker Rov). Others maintain that it is enough to have only six people who are now praying joined with four non-praying individuals to constitute a minyan. This is based upon the premise of rubo kekulo, a majority is considered as the whole. (Machaze Avrohom O.H. 2: 9, Chelkas Yaakov 2: 138, Mishna Berura 69: 8, quoting Magen Avrohom, Minchas Yitzchok 9: 6, Toras Chaim p. 56, Yalkut Yosef p. 265, Halichos Yosef p. 83)
Some Poskim maintain that a minyan of rubo kekulo is considered a minyan for shacharis and mincha which contain the chazoras hashatz repetition (in which the non-praying members participate by listening) but not for ma’ariv. (Orchos Rabbeinu Chazon Ish ch. 160 p.51, See also Ishei Yisrael 34: 9.)
Horav Shlomo Miller’s Shlit”a opinion is that it is advisable when possible to wait for a minyan of ten men who have not yet davened or to attend a shul that fulfils this requirement, however, since the Shechina is present when a minyan is in shul, in cases of hardship you can rely on the lenient opinions.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a