Migrations, like pregnancies, seem to be essentially set in progress. With a cap of 50 and a population of 49, a migration of any number of dwarves can be sparked. There's no reason that when the first dwarf enters on the side of the map that any of the other dwarves (and bear in mind that the next one along could be a spouse of child of the one just arriving) the others will not continue to arrive.
IIRC, the first few migrant waves of your fort are actually pre-set to arrive right from the beginning (more or less), so that with a cap<7, immediately broken by migrant wave zero (your initial founders) you still get the migrants. (Assuming the various other migrant conditions are met, probably, such as general fortress attractiveness.)
Someone else will know whether this might extend towards later waves, so that after a migration arriving in a previously 49-dwarf fort you still get the next migrant wave and the one after that, but that's not something I remember being mentioned.
Personally, I currently set my cap to 300 (and children cap at 50% more, as well). Not that I often reach it, as I find that there's new versions to play with and old fortresses end up sitting idle and uncared for, never run again, long before they reach that cap. I'm surprised I'm still playing a .21 fortress, at the moment, given I've had .25 downloaded and expanded onto my HDD almost since the day it happened. I think it's because I've got a bit of a midi-project going on (a 1Z waterfall and some compliant terrain giving a promise that I might make a shallow-flooded lake on the lower level), and I'm trying some new tricks with tame animals.
(Pro tip: Yes, you can set out a whole lot of 1x1 pastures all around the outside of the fort and fill them with the copious number of guineachicks that would just otherwise squabble and fight in the main bird pasture, but when you get attacked by macegoblin squads (handily detected and brought to your attention by said chicks),
you can't tell the difference between them. Also, unless you immediately seal the entrance, and/or done other alert/burrow-based trickery in order to restrict your civilians, people wander outside en-mass to try to re-pasture the guineachicks who have started to stampede from their pastures in response to the marauding goblin squads.
)