#define MAX_TEAM_MEMBERS 20                     //Change to yours...

//OnFilterScriptInIt
SetTimerEx("AutoBalance",600000,true,"d",playerid); //600000 = 10minutes.

forward AutoBalance(playerid);
public AutoBalance(playerid)
{
      //Use 'if' statement to see how many people on team, something like this
      if(gTeam[playerid] > MAX_TEAM_MEMBERS)
      {
              SetPlayerTeam(playerid,2); // Something like this should work.
      }
      return 1;
}

new TeamPlayers[ <Amount of teams here> ];

public OnPlayerRequestSpawn(playerid)
{
  for(new teams; teams < [Team Amount]; teams++)
  {
    if(teams == GetPlayerTeam(playerid)) continue; //Or gTeam[playerid], or w/e you use.
    if(TeamPlayers[Player Team Variable] > TeamPlayers[teams] + 3)
    {
      SendClientMessage(playerid, COLOR_RED, "You can't join this team as it would unbalance the game!");
      return 0;
    }
  }
}