Some maps have a limited amount of spawn points, which makes you unable to have more than 8 or 16 players per team. Search Google for "32 players CS maps", or else Austin has written a little app that enables you to edit the spawn points in your CS maps and save them back to disk (it's called MapHack). There's also a metamod plugin version of it that enables you to do that visually while in game.
*edit*
roger that KWo
