How to count using Interrupts
Make it a volatile global variable; declare and initialize it outside all functions. Then you increment within your ISR and print the modified value in main(). Something like:

static volatile int count = 0;

isr(){
  count++;
...
}

void main(){
  debug_print(count);
...
}


The volatile keyword is to prevent your compiler from optimizing away the variable, since its not seen to change in the normal program flow. 

EDIT:
Declaring the variable static, like Lundin suggested, restricts its scope to this particular file, so that the variable is not visible outside this source file. You should use this only if the ISR and main() are defined within the same file.