Thinking sideways - are you by chance using RADIUS authentication with something like WPA2-Enteprise mode?
If so, you may be able to export all your authentication events (hint, with NPS, it is event ID 6272), parse the data in the authentication log to extra out the AP, and then use something like Excel to create a count for successful authentication by AP.
But the analysis will take quite a bit of time - could be cheaper to buy an extra AP.