As far as i know, minimum for emergency is determined by the number of seats (1:50) and the number of crew members needed to open primary doors. Though most airlines have "service" minimum. Lets take an EL AL 3 class configurated 744, and a 2 class 757-200 as examples.
752 - An EL AL 752 carries just under 200 pax and has 6 primary doors (or three pairs 1L-R, 2L-R, 4L-R). Using the 1:50 key, then 4 crew is enough. Since there are 6 doors which 3 crew member can easily open (one for every pair of doors), then the minimum for emergency is 4. EL AL adds two more crew members, thus 6 is the minimum crew for service on EL AL flights.
744 - An EL AL 744 carries about 410 seats and has 12 primary doors (including upperdeck 1-6/L-R). If the number of crew was only determinded by the number of seats, then 9 would be enough (using the key of 1:50). As already mentioned a 774 has 12 primary doors (or six pairs) but since one f/a would have difficulty operating both pairs because of the distance between each pairs then in ultra wide bodies (777, 742, 744) each door needs one crew member. Thus 12 is FAA/CAA minimum for a 744. EL AL adds 6 more F/A to a 3 class configurated 744, making the EL AL minimum 18. (2 Pursers, 2 First, 6 Business, 8 Economy).
Back to the question of the thread. I find 18 crew members enough for a loaded 744. I dont think there needs to be more than 18 on a 744. Anymore will not be helpful for the crew. There are a specific number of trolleys that can fit in a plane and a specific number of aisles and zones. In econmy if you add more than 2 f/a to a zone (there are 3 C, D and E) and one f/a to each of two galleys then you will just be adding caos. On flights that i flew, the only time extra f/a were needed were when there was an excessive number of special meals to hand out. In those cases crew from the forward (first and business) usually come to help out. I really feel though that more crew then need is not helpful. Sometimes service that is too fast is not so good for the pax either.