According to the sometimes-reliable font of all knowledge Wikipedia they've got 54 sets and the
top of this page on FGW's site says 53; your count only comes to 44- which is closer to what the bit at the bottom of that page says (43!)
What's the real number?
Sorry, missed a sub-fleet
Add to my numbers 10 2+8 high density sets allocated to Old Oak Common. A total of 54 sets. That's for the start of 2011, possibly it's changed now to the 53 sets mentioned by
schnellzug and FGW, but I'm not sure how twenty 2+7 high density sets works when they essentially have the 401xx catering vehicles allocated to them, and there's only nineteen of those vehicles. The abrail site linked a few posts above backs me up on my numbers.
So just for completeness, to my knowledge the sets are arranged thus:
LA01-LA16: 16 x 8-car long distance sets.
LA60-64, LA71-75: 10 x 8-car high density sets.
OC30-OC39: 10 x 8-car high density sets
OC40-OC57: 18 x 7-car high density sets
Details taken from the Platform 5 Combined Volume.