I'd like to once and for all sort out all the silly population cap nonsense once and for all.
After starting with 7 dwarves, you will, even as soon as summer of your first year, recieve immigrants. These immigrants, arriving in waves rarely more then 24 short persons per wave, will continue to bolster numbers until a population cap of 200 dwarfs are met or until your computer commits Harri Karri, having gone insane from processing the minds of so many dwarfs.
The easiest way to curb this inevitable destruction of your computer is to set the Population cap in your DF init file lower, where upon the waves of dwarves (That can range much larger then the previous stated 24 persons) will continue until the limit is met or surpassed, after which the waves will stop.
There for, if you set your population limit to 8, you would recieve one immigration wave, which could be much larger then the one dwarf needed to fill your limit. However after that, unless deaths in your fortress force your population under that limit, you will not recieve any other immigration waves.
There is no way to ensure that you only get the amount of dwarfs required to fill your limit (1 in the example above for instance) as when the limit is not met immigrants will arrive in waves, which as I stated can, and will be much larger then you want. To this end I suggest any kind of lava/bottemless pit/starvation room/arena based means of population control.
I hope this clears up any of the "ZOMG POPALITION CAP IS BUGGED LOL!1!" and/or "Oh bother! Watson it does appear that I have recieved far to many dwafs in my immaculate, perfectly summetrical fortress, I shall complain about this immediatly to the fine high class gentlemen of the forum!" Shenanigans.