how many class can contain a header

Answer 1

As many as u want

Answer 2

you can add as many classes as you want . To state it in a professional way and for best practice, Add just one super/parent class and the rest should be a sub/child class.

Answer 3

One Header file can include as much classes/definitions as you wish.
Note that you have to include your *.hpp in your *.cpp which means you not even need the same name here!

In sake of readability you should only put classes/definitions in the header file they belong...