I guess it must have just before the Valiant fleet was withdrawn and the Mark 1 Victor bomber squadrons disbanded, some time in 1964?
That would give 9 Vulcan, 4 Victor B1, 2 Victor B2, 3 Valiant B1, 1 Valiant recce and 2 Valiant tanker, 21 squadrons in total? Plus 2 OCUs, a Development Unit and a radar recce flight?