Q. Why does prayer require a minyan?
The definition of community for this purpose is ten, since the Torah (e.g. Gen. 42:45, Ex. 18:21, Num. 14:27) regards ten as the basic social unit.
The Mishnah (Megillah 4:3) lays down the prayers that require a minyan; they include Kaddish, K’dushah, Bar’chu and reading the Torah. All are deemed d’varim shebik’dushah – “passages (or prayers) of sanctification”.
The minyan rule has not only spiritual but social benefits in that it creates a sense of cohesiveness and community.
The sages assure us, however, that even without a minyan one is in the presence of God and a person may (and must if necessary) meditate and pray alone (Avot 3:6).