Thanx for responding!
We can't guarantee that 27 pilots died. As someone mentioned in an older post string on this subject, some of the pilots may have opted to retreat early. [Perhaps some of them had technical problems with their fighters.]
In any case, even though we see 2 X-Wings, a Y-Wing & the Millenium Falcon fly away in the exploding Death Star shot, we can still guess that other ships may have flown off earlier [or off screen].
I can re-watch the sequence & get back to you with a more definite answer but I'm guessing, off-hand, that only 6 Rebel pilots were confirmed blown out of the sky plus 1 was downed for a crash-land death (Red Leader).
As Darth Chaltab mentioned, Luke & Wedge are confirmed survivors. The fate of the other 21 pilots (20 if you count Pink Five...ha, ha) was not clearly documented in "A New Hope".
Best wishes,
sidemachine