connect wx.EVT_PAINT with method that refreshes your filecount
self.listBox.Bind(wx.EVT_PAINT, self.on_list_update)
def on_list_update(self, event):
event.Skip()
self.findchecked = self.listBox.GetChecked()
self.filecount = self.listBox.GetCount()
self.filecountdisplay = str(len(self.findchecked)) + "/" + str(self.filecount) + " Files"
self.inlistDisplay = wx.StaticText(self.panel, -1, self.filecountdisplay,(160,215))
self.Refresh()