How many numbers are there between 75 and 100?

Infinitely many.

For example,

partway between 75 and 100 is 90

partway between 90 and 100 is 95

partway between 90 and 95 is 94

partway between 94 and 95 is 94.5

partway between 94 and 94.5 is 94.001

partway between 94 and 94.001 is 94.0005

and so on.

And between any of these pairs of numbers are infinitely many numbers.