I can think of two that have at least 200 runners for both sexes. One is Green Hope (http://www.usatoday.com/sports/preps/xcountry/2008-10-14-big-teams_N.htm) in north carolina and of course York.
Any other schools with very large cross country rosters, and are these the schools that typically make it to NXN?
Lake County, Colorado back in the day would get close to 50 (boys and girls) out-this in a high school with fewer than 400 students. Had more male runners than our football team had players.
We just had a thread discussing this.
How large are those schools? My high school had about 1,000 students total (~500 male / ~500 female) and we sometimes had difficulty putting together a varsity squad. We sometimes had to borrow guys from the soccer team.