new g_MaxPlayerOnline = 0;
public OnPlayerConnect(playerid)
{
      g_MaxPlayerOnline++;
      return true;
}
stock GetMostPlayersOnline()
{
      if(g_MaxPlayerOnline)
      {
            return g_MaxPlayerOnline;
      }
      else return 0;
}

// Create two arrays holding numbers, starting with0
new g_MaxPlayerOnline = 0,
      g_TotalPlayerOnline = 0;
public OnPlayerConnect(playerid)
{
      // Total players online raise
      g_TotalPlayerOnline ++;
      // if there are more players online *now* than it before then we or raise g_MaxPlayerOnline or set it to the same number which g_TotalPlayerOnline holds, which is also only 1 decimal far away, because only one player is connecting at this point.
      if(g_TotalPlayerOnline > g_MaxPlayerOnline)
      {
            g_MaxPlayerOnline = g_TotalPlayerOnline;
      }
      return true;
}