The technical answer is that a CBG (Carrier Battle Group) is made up of certain kinds of needs for the carrier. Ideally, the planners would want an Aegis cruiser to accompany each carrier, and then they'd want a range of destroyers to form a screen. Now, not all ships can stay on station full time. Some have other duties, some are due refits, some break down or need to rotate back to the homeport for crew retention.
So, it's a complicated mix. Right now there's a mix of Arliegh Burkes and Spru-cans mixed with OHPs. The size of the screen will reflect the location of the CBG, but it will also reflect if there are 'high value targets' such as AO, AOR, AER, or the like deployed with the CBG (as is common in the eastern Med, or in some Asian deployments).
Where this gets complicated is during flight ops. The carrier turns into the wind from whatever base course the CBG was on, and accelerates to whatever speed "full" is for flight ops (it can vary). The CBG rearranges medium-radically when that happens. If one has stores ships assigned, they can often form a sub-group (the better to not have to get to carrier speeds), which will need additional escort vessels. Not under flight ops, every body meanders back in to the steaming formation that the CBG commander has set.
I cannot remember where I saw it, but there was a wonderfull word picture of a CBG using DC as a center point, and giving geographical references for where the escorts would be (near as Harper's Ferry, far as Erie, PA; S-3 Vikes out to most of St. Louis . . . ) Dang it, now I'll have to rummage through my bookcases to see if I can find that . . . (probably better than watchin boring olympic coverage <g>)