(See also these two Reddit comments by brianmcn)

If you run a function using this command:

/function foo:bar if @e[tag=test]

the function foo:bar will only be run if @e[tag=test] matches exactly one entity.

Correspondingly, this command:

/function foo:bar unless @e[tag=test]

will run the function only if @e[tag=test] matches no entities or more than two entities.