It means "the number of times something happens every second".

A number of them happen to be in the wrong party.

If the stock began trading even higher, a number of things could have happened.

Any number of things can happen to turn around the performance.

By then, a number of things were happening at once.

A number of things happened then, following each other too swiftly for thought.

My number just didn't happen to be up that time.

"And that number for us happens to be $40 a month."

A number of things happened at nearly the same time.

He had only one telephone, its number happened to end in three.