If you want to count how many calls IVM answered, it's fairly easy to use the logging feature in the OGM properties window of the Answer OGM to make note of each time a call is answered. You can also create a plug-in that will store the number of calls in a more user friendly way and with the possibility of statistical analysis (e.g. calls per hour, day, week, year, on Monday, on Wednesday etc.)
On the other hand if you want to record every call that rings IVM (including those not answered), then that's an entirely different story and I'm not sure how you would go about doing that.