[Hiddos]

A better idea:

pawn Code:

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;
    }
  }
}

Just a small sketch of a situation, unsure if I made some mistakes in my code though.