18 Fighters normally comprises a Squadron, not a Group. A fighter group would normally contain 77 fighters (3 x Squadron of 25 (16 on missions, 9 for spares). The unit is later called the 511th Fighter Squadron.
The 511th Fighter Group has 18 of the new P-51's (Mustang) long range fighters.
General. Britt says "Count the ones that got back not the ones that are missing" (8).
When the 918 returns, Major Stovall says they are missing 19 aircraft.