A school can represent up to three countries plus four additional (independent) delegates. Each country can have up to 10 delegates (one per committee), though any delegate assigned to OPEC from a non-member country may be changed to an independent delegate to represent an entity that makes sense in that group.
While you can request specific countries, the three you end up with must be from different areas of the world and must not be countries your school has been assigned within the last two years. The order of the requests is taken into consideration, but is not the sole deciding factor. You'll get a reply within a few days telling you which countries and independent spots you've been assigned.