Thankyou for your pic OpenMaw. At least I appreciate what you've tried to do to get through to some very awkward posters on here.
But shall I tell you why I think there are 3 engineers? Because of the FRAMING OF THE SHOT.
If it was 2 + David, the pic would be cropped so David would be closer to (our) right frame edge. You wouldn't have a shot with action happening on one side, David in the middle and nothing on the other. It's all about BALANCE in the shot! Basic camera law.
ALSO, 4 SARCOPHAGI - my logic is, 3 Engineers awaken from these, the 4th sarcophagus is reserved possibly for David in his new role?