I had a hard time to find the correct OID and ended with doing a snmpwalk and try to find the right OID with a grep…
But i think i made it 🙂

Crosschecked on 2 5760 Clusters and i got the correct count on both, also tried to reboot some APs and the count seems correct to me.
Note: I use 3.6.1 so maybe you get a different result with another software version.

1.3.6.1.4.1.9.9.999999.1.1.1.11.0

Interesting is that i found two other OIDs that report the same value.

1.3.6.1.4.1.9.9.999999.1.1.1.17.0
1.3.6.1.4.1.9.9.999999.1.1.1.18.0