Most places use the requirement of 1 cabin crew for each 50 pax. In the past it used to be related to the number of doors, but it's a been a while since many places worldwide changed this requirement for one CC for each 50 pax (with a lot of lobby by the airlines of course).
Just look at the airlines flying the B737-700 and A319. Most use only 3 cabin crew.
The 787-8 at QR is configured for 254 pax, so the minimum number of cabin crew is actually 6.