Eh, I've worked with a lot of college radio (although admittedly mostly in the northeast) and I wouldn't say sheer numbers are all that great a metric.
First, if you're at a big university with 10000+ students, it starts getting easier to have lots of interested students who'll show up just because of the law of averages. Concordantly, if you don't have a lot of students, it can get harder. Wellesley College has a lot of trouble getting a full DJ schedule because they're tiny. At my college, Hobart & William Smith, we'll probably never fill all 90-odd hours available for student DJ'ed shows since there's only 2000 students...getting even 5% of the entire campus to do any one activity is next to impossible unless you mandate it.
Second, if your college has a radio curriculum, or a radio-related one (journalism, engineering, etc) it's also easier to have more interested students. And vice versa if your college doesn't. Boston University has no radio curriculum, but DOES have a very strong journalism program. It also has a lot of decent-to-good sports teams for aspiring sportscasters to cover. Couple that with a 15,000+ undergraduate population and it's no surprise they have 150+ students involved at WTBU every semester. WMLN at Curry College, I am told, rarely has trouble filling the roster despite a minimum Class A signal on a crowded frequency that can't be heard in most of Boston, because they have a pretty serious radio curriculum to attract radio-minded students.
Third, what type of situation you have at your studios and your signal size can make a huge difference. Emerson College has WERS, the largest signal for "college radio" in Boston by far, and one of the larger signals in the market, period (it's a Class B). That sure doesn't hurt their ability to pick and choose the cream of the crop from the zillions of applicants they get every semester. WTBU, in the pre-web/Part 15 days when the lack of a signal really hurt enrollment, moved to new studios with new gear...and the studios used to belong to highfalutin' WBUR...and enrollment spiked up for a year or two thanks to that. WBRS moved into new digs that were largely perceived as a big step DOWN from their old home (despite new equipment) and enrollment suffered.
Fourth, what sort of oversight/relationship with your college will also have a major impact on your enrollment. Whether or not you have professional staff, whether or not that staff is also teaching classes (which is a HUGE distinction on the station's "place" within the colllege), and just how much attention the college administration pays to the station (i.e. "flying under the radar") will all dramatically affect the ratio of number of total students and number of "active" students.
Finally, enrollment at a station is also cyclical. There are good years and bad years. 1990-1997 were generally not good years for college radio. Then webcasting started hitting the mainstream and students got more media savvy, and there was a burst between 1998-2005 or so. Afterwards, things kinda dropped off again, as incoming students found radio passe after growing up with high-speed internet and podcasts. I think things picked up somewhat with the 2008 presidential election as "the youth movement" again expanded their media savviness, but it's since trailed off just as quickly. Individual schools can buck the trend...a few hot years with a popular sport winning a lot and you might see a spike in sportscasters, for example. But sometimes it just happens to be that you can't lure students in with free beer, and sometimes you can't beat them off with a stick.
FWIW, I have noticed that there is something of a constant, where you rarely will have more than 5-15% of your total number of students be "actively" involved in roles beyond the "once a week DJ" archetype...doing things like management, sportscasting, news reporters, etc. Obviously there are exceptions and that rule is variable on a host of abovementioned factors, but you don't generally see more than a handful of real hardcore radio geeks that thrive on "living at the station" these days.
