[Frank Mackey]

I think any of those answers, except for "I cannot remember", works pretty well! Assuming that you are over 18 at least, I would also suggest that you just smile while looking at her eyes, make a short pause to turn them away as if you were considering the body count, then stare at her again - now smilling a bit more ironically - and just say "Really, now that I think about it, I think I have lost track of the number since I was fifteen..."