"A large number of " is talking about a specific group (definite reference).

Whereas "Large numbers of" is not talking about a specific group (indefinite reference)

Over there is a large number of soldiers.

You cant say "Over there is large numbers of soldiers"

It is the same with a lot of things...

The library has huge collections of music.

The library has a huge collection of music I really like.

Although meaning is very similar, I wouldn't worry too much.