With UT2003/2004, the default assumption regarding Teams was that they always consisted of two sides. Then there appeared new community made team games such as TDM4 and CTF4 that included more than two teams. The number of teams is important to MapMixer because it helps determine team player count balancing (making sure teams contain even numbers of players).
MapMixer now keeps a silent list of games types that contain more than 2 teams. Currently it contains entries for the gametypes TDM4 and CTF4. You can edit these (or add more) yourself manually in the MapMixer.ini file if you need to - it appears as:
[MapMixer.MapMixer_GameTypeManager]
TeamOverrides=(sGameClass="OLTeamGames.OLCTFGame",tTeams=4)
TeamOverrides=(sGameClass="OLTeamGames.OLTeamDeathmatch",tTeams=4)
Generally speaking, you will not need to edit these but if you need to for some
reason, then you can.