There are no API endpoints to see who is watching the stream. There are some apps that use the list of chatters and assume they are viewing the stream, but really that list only represents users connected to chat and logged in to Twitch, it does not mean that they are actually watching the stream as they could be connected to chat through a 3rd party chat client, or Twitch’s popout chat, or have the stream paused, or even be a bot and not capable of watching the stream at all.