When each map is designed, the author will usually (if they're serious) specify a recommended minimum and maximum number of players for the map. Then UT2003 can make sure you at least have the minimum number of players in a match, spawning bots to make up the numbers. In Multiplayer games, only the maximum number of players specified for the map can connect at one time. MapMixer refers to these settings as the Map's Player Counts.
MapMixer can override the recommended player counts per map and provides two ways of doing this.
If you choose to enable Player Matching then you can have MapMixer adjust the number of players in a match depending on the Player Count settings of the map.