Telegram with Numix never display the notification number on systray. I am using it on XFCE4. in details I can see that themes like for example Adwaita or Mint -X or Mint-Y or Arc displays properly that notification number. So there must be missing svg or metadata definition which drives Telegram to think it is dealing with a monochrome theme? here is with Mint-Y  Here is with Numix (you have to restart telegram to see the change) 