Learn English – Synonym for “proud parents”

Two suggestions:

To fill up that vat you'd need a great many tennis balls.

To fill up that vat you'd need countless tennis balls.

The phrase a great many is often used to convey the idea of an awful lot; with a great many hits on Google books (well over a million, in fact).

Countless doesn't really mean uncountable, but suggests that it might take a truckload of time to count all the tennis balls.

When you don't know how to use your firearm, you become a menace.

As used here, menace is a noun meaning:

A person or thing that is likely to cause harm; a threat or danger