If you're using LNP the pop cap is exported in the UI, otherwise you'll have to change it in the file manually. I let the first two wave come as they like, and after the second one I exit DF and set the pop cap to the current pop + 10 and then restart DF. I repeat this until I come to within 10 of my target pop, at which time I set the target one and leave it at that. The strict pop cap is set a bit higher to allow for natural growth (kids), and, I assume, petitioning visitors (never managed to get any visitors so far...).
Setting a low pop cap has side effects, since invasions are allowed only when the pop reaches 80 (unless the parameter is changed to a different level, or you enable invasions due to wealth and/or exports), although there are reports of invasions prior to that for unknown reasons. Baby snatchers and ambushes are supposed to be back (haven't seen either), and are not counted as sieges. Kobold thieves are not sieges either.
Also, titans are allowed to attack at a pop of 80 as the default (although I think there is an export trigger as well), and a certain kind of migrant may show up only on a pop of 80 (I haven't found any parameter for that).
To become the mountainhome you'll need a large pop (I think one of the requirements is to be a metropolis).
Also note that black tower invasions disregard all limits. They can arrive in the first summer (just make sure you have a door, lock it, and turtle).
You COULD make alternative uses of excess migrants, of course. I wouldn't drop the migrants to just necromancers of I wanted to create an army, though, as necromancers are fairly weak, so the migrants will probably kill them if they don't have any support (or the migrants are suitably pre processed).