A group has more protection against harm than an individual, as in Her parents won't allow her to date but do let her go to parties, saying there's safety in numbers. This phrase comes from the Latin proverb, Defendit numerus, presumably alluding to a military situation. It was first recorded in English about 1550.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.