In Judaism, for certain prayers to be devotionally valid, a minimum of ten adult men [known in Hebrew as a 'minyan'] is required to be present. There is a good wikipedia article giving details of the occasions where this is requisite. A foonote states --

"Maimonides (a preeminent medieval Spanish, Sephardic Jewish philosopher, astronomer and one of the most prolific and influential Torah scholars and physicians of the Middle Ages) writes that "The prayer of the community is always heard; and even if there were sinners among them, the Holy One, blessed be He, never rejects the prayer of the multitude. Hence a person must join himself with the community, and should not pray by himself so long as he is able to pray with the community." Mishneh Torah Hilkhot Tefillah 8.1

Note that the final clause provides a let-out for times that a minyan cannot be gathered for any valid reason, or even if the worshipper is for some reason in a solitary situation.

~M~