I started with 15-20 adults. Through initial aggression I usually end up with 12-15 fish. It doesn't take ling before there are 30+ Tropheus in the tank.
When I grew them out from Juveniles the initial pruning didn't happen. But I am impatient so I would get adults if I were doing it all over again.
I kept mine with Julidochromis and Spathodus erythroden with good success. The tropheus will dominate the upper to mid-level of the aquarium so if you want something with them in that area it will need to be pretty aggressive like a Petro.