The developed test cases are coverd all the functionality 
of the application we can say testcases are enough.

If u know the functionality covered or not u can use RTM.

min number of test cases = no. of inputs * 1.6

and for enough test cases you need to write the test cases 
which are coverd all functionalities in your application 

and for maximum no. of test cases you need to write test 
cases all aspects of validation and functionality testing.

Number of test cases can be determined by knowing the 
number of validation point for the given module.

we can make sure all the requirements is covered in test 
case by reviewing the RTM