"Overlord" by Hax Hastings:
The entire allied air strength in the West, 6th June 1944, which included:
RAF Bomber Command
ADGB (Fighter Command)
2nd TAF
8th USAAF
9th USAAF
Was 9901 a/c of which 7774 were effective strength (crews available). Thats for the entire western front, the bombing of France and Germany, Air Defence, all of it.
Allied OOB for France consisted of the RAF 2nd TAF with @ 1254 combat a/c total (bombers and fighters), and USAAF 9th TAC with @ 2339 combat a/c total (bombers and fighters). Of that, about 2790 were effective strength (had crews).
"On 6 June 1944, the Allies had a total of 13,000 aircraft ready to support the Normandy invasion"
Maybe if you included every single a/c in Britain, from every RAF, RCAF and USAAF command, including liason and transports (C-47s, Piper Cubs ect), and only gave totals, as opposed to operational, you might get a bloated # like that.