From Quake Wiki

The info_player_coop entity functions the same as the info_player_start entity, with the exception that it is only used in coop mode. In maps that support coop, there are usually four of these, since the player limit for standard NetQuake is 4. The regular player start point is also used in coop, so in reality, only three are necessary to support four players spawning at the same time without telefragging.