Code:
struct MyStruct
{
  /*members*/
} MyArray[100];
This is a static sized array, it always has 100 elements. You can use the sizeof trickery to instruct the compiler to get that information for you.

If you're talking about knowing how many of the array elements are "used", then that's up to you to determine yourself. There's a way I can suggest, let me know if this is what you mean and I'll explain.