Yup! It sure is. The best way to do it would be with eventhandlers. Check the online com ref here for more info.
Basically, you can set it up so that every time one of these objects is killed/destroyed, a script or line of code is run. In your case, that line of code could just add to a counter of how many have been killed. You could do that by putting this in the init field of each target:
this addeventhandler ["killed", {HIT_TARGS = HIT_TARGS + 1}]
Then in your init.sqs, start the variable HIT_TARGS out at zero:
HIT_TARGS = 0