Please note that what you are planning to do is not permitted according to my understanding of the Third Party Developer Agreement and will require previous written permission from Twitch.
If you have such permission, your best bet to find out who is watching who will be to connect to chat and see who is chatting - this number however will be heavily skewed as “being in chat”, “chatting” and “watching the stream” are very, VERY distinctive things and do not necessarily correlate. The latter you cannot get from Twitch, the second is gatherable if you are listening to chat when it happens and the first one is not available from officially supported sources.
Thus, as much as I want to be welcoming and support new people working with the Twitch APIs, I cannot recommend this specific type of interaction.
(I am not a Lawyer and this is not Legal Advice.)