Hi Sahana,
A Mandatory prompt will Not behave differently for a String prompt vs a Number prompt.
The only way both can run without entering a value is when a 'Default Value' is set. For a string prompt this can be set to a 'space character' whereas for a number it needs, of course, a number.
Open the report in the CR Designer > Expand the Field Explorer > Right-click the prompt > Edit > Scroll down and look for the option 'Default Value' - you should perhaps see a 'space' entered in here.
-Abhilash