It's very much a play style issue, and you'll find what fits you in time.
- Only starting 7: You get to know them well. On the other hand, everything takes a lot of time because of the limited number of hands. Also, each dorf has to have multiple professions to cover all the bases you want to cover, and that results in a lot of micro management as you need to shift jobs around.
- Bring 'em on: Gives you the workforce quickly, and also fresh bodies to throw to the invaders. If you think defenses are for wimps this is probably your option.
- Gradual increase: I usually save each season and increase the pop cap to 10 above what it currently is, usually resulting in 6-9 migrants, which I find is a nice rate (cutting this off when I reach what I find is a balanced level).
- Playing a truly dead civ: You get your starting 7 plus two waves, and nothing more. Nor do you get caravans, trade liaisons, a monarch, or foreign diplomats. Note that getting a truly dead civ is non trivial, though (and shouldn't be confused with a dying/struggling/... civ, which is rather common).
Note that unless you change parameters, megabeasts and goblins won't attack you until your pop is 80, while black towers have no qualms about sending a greeting party your way the first summer. Also, you need a pop of 50 to get a mayor (which isn't necessarily a good thing, due to mandates).