To check the "only-16-squadrons-at-one-time-by-rotating" theory I did count the squadrons that were at one time in No. 11 Group.
July 10th 1940
Hurricane 13
Spitfire 5
Blenheim 3
Defiant 1
http://www.battleofbritain1940.net/document-22.html
August 8th 1940
Hurricane 14.5 (No. 85 Squadron is listed twice as the two Flights were at different stations, No. 1 RCAF is operational on August 17th when No. 41 Squadron was already transferred back to No. 13 Group)
Spitfire 5.5 (No. 41 Squadron returned to No. 13 Group on August 9th)
Blenheim 2
Defiant 0
http://www.battleofbritain1940.net/document-28.html
September 7th 1940
Hurricane 14
Spitfire 7
Blenheim 2
Defiant 0
http://www.battleofbritain1940.net/document-44.html
If we assume that 16 squadrons operated on 100 octane at one time, by summing up the Spitfire, Hurricane and Defiant* squadrons in No. 11 Group we get:
July 10th: 19 Squadrons
(84% on 100 octane)
August 8th: 20 Squadrons
(80% on 100 octane)
September 7th: 21 Squadrons
(76% on 100 octane)
*Blenheim only used 100 octane fuel for better take-off performance when heavy loaded, which was not required in Fighter Command as they didn't carry bombs.