new UsersConnected; // We create a variable that counts the players.

public OnPlayerConnect(playerid)
{
    UsersConnected ++; // We add the value whenever a player connects
    new string[25];
    format(string, sizeof(string), "Players online: %i.", UsersConnected);
    TextDrawSetString(Text:text, string); // It changes 'Text:text'.
    return 1;
}

public OnPlayerDisconnect(playerid, reason)
{
    UsersConnected --; // We diminish the value whenever a player connects
    new string[25];
    format(string, sizeof(string), "Players online: %i.", UsersConnected);
    TextDrawSetString(Text:text, string); // It changes 'Text:text'.
    return 1;
}

new data[30];
format(data, sizeof(data), "players online: %d.", CountingOnlinePlayers());
TextDrawSetString(Text:text, data);

stock CountingOnlinePlayers(){
    new count=0;
    for(new player, user = GetMaxPlayers(); player != user; player++){
        if(IsPlayerConnected(player)){count++;}
    }return count;
}