word choice - a large number of/ large numbers of, What's the difference?

Tests are hard to solve for large number of users.

Checking the similar usage of such expression, I think there is something wrong with the sentence. What's the best and correct? a large number OR large numbers OR even something else!

Answer

These are both correct:

Tests are hard to solve for a large number of users.

Tests are hard to solve for large numbers of users.

The second sentence has a slightly more general connotation, like "users in general", whereas the first probably implies users of a specific application.