If you need to know how many times an external interface method is called, use some mocking framework. See also
What C# mocking framework to use?[
^].
These frameworks provide means to assert how many times and with what parameters you expect the method to be called.
If the method is an interal method of the Unit-Under-Test (UUT), then you need to add some internal test code.
In any case, your main challenge will be the decopling of the dependencies. See
Inversion of Control[
^] and more specifically e.g.
Dependency Injection[
^] so that you can plug the mocked external code to the UUT and maybe plug in some internal test code to the UUT.
Cheers
Andi