I like to say that 1 inch of fish for every 5 gallons is a good rule of thumb. There are other circumstances to be taken into account too, but for a fowlr, I think it works.
Some people say 1 inch per 10 gallons in salt is the way to go, but I find that to be really limiting.
There are also other factors to take into account, like the maximum length of the fish and its ability to turn in the aqaurium, so depth should be taken into account, and the structuring of the live rock as well.
Triggers are cool fish, but i just dont care to keep them, ever. There are some that are considered mild when it comes to aggresiveness, but I hear horror stories of waking up one morning to find the trigger alive and everything else dead.