"Hellcat Aces of WW2" by B. Tillman
"Most nightfighting Hellcats kept the standard six gun .50 cal armament, most units experienced jamming problems with their cannon."
The book lists at least a half dozen Navy and Marine Night Hellcat squadrons, from the Phillipines campaign to Okinawa to the strikes on Japan. It does not say what unit specifically used the 20mm varient though. Just that they flew F6F-5(N)s.
It uses the term "a modest number" of Hellcats had the 20mm armament, but gives no exact #. My "guess" would be several hundred served in the Pacific. Thats "modest" for the US Navy in 1945, considering the huge # of CAGs deployed by wars end.
For what its worth.
For the most part 6 .50s would have lit up anything in the IJN/IJAAF inventory when tracked and hosed by a Night Hellcat. The 20mm option probably did not increase the kill probability all that much.