[Basicz]

pawn Code:

new
    iArmyCounts,
    iRobberCounts
;

public OnPlayerSpawn( playerid )
{
    if ( GetPlayerTeam( playerid ) == TEAM_ARMY ) iArmyCounts ++;
    if ( GetPlayerTeam( playerid ) == TEAM_ROBBERS ) iRobberCounts ++;

    return 1;
}

public OnPlayerRequestSpawn( playerid )
{
    if ( GetPlayerTeam( playerid ) == TEAM_ARMY )
    {
        if ( iArmyCounts < iRobberCounts )
            return !SendClientMessage( playerid, -1, "Please join robbers team." );
    }

    if ( GetPlayerTeam( playerid ) == TEAM_ROBBERS )
    {
        if ( iRobberCounts < iArmyCounts )
            return !SendClientMessage( playerid, -1, "Please join army team." );
    }

    return 1;
}

pawn Code:

return !print( "lol" );
// ==
return print( "lol" ), 0;
// afaik xD

Hope this one works, and helped you.

EDIT: I got what do you meant.

pawn Code:

new
    iArmyCounts
;

public OnPlayerSpawn( playerid )
{
    if ( GetPlayerTeam( playerid ) == TEAM_ARMY )
        iArmyCounts ++;

    return 1;
}

public OnPlayerRequestSpawn( playerid )
{
    if ( GetPlayerTeam( playerid ) == TEAM_ARMY )
    {
        if ( iArmyCounts >= 5 )
            return !SendClientMessage( playerid, -1, "Team army is full." );
    }

    return 1;
}