[Hiddos]

pawn Code:

new TeamCount[2];

public OnPlayerRequestSpawn(playerid)
{
  if((gTeam[playerid] == TEAM_CT) ? (TeamCount[0] > TeamCount[1]+3) : (TeamCount[0] < TeamCount[1]+3))
  {
    SendClientMessage(playerid, 0x00ff00ff, "Unable to spawn, the teams aren't balanced!");
    return 0;
  }
  TeamCount[(gTeam[playerid] == TEAM_CT) ? (0) : (1)]++;
}

You'll need to work on something to reset this variable again, for example when a player joins a different team or disconnect, set the specific team's count minus one.

Also, this is just a crappy example. I guess it's best to make your own system