The only limit I know of for track rosters is the number of people allowed to compete at the conference meet (I think it's 35 people). I believe this is the case in order to try and keep heats at a reasonable size so the meet can go smoothly and keep the school budget under control (I was told this while I was doing a class project on the air quality at the Harvard indoor track during the indoor Heps of 2005).
I don't know if there a competing-team-number limit for XC. You distance afficianados will have to tell me, lol.