Since the game is run on Wii U it rules out everything but actually grabbing screenshots when it streams to your PC. But looking into it I feel that it’s definantly is doable - given that the existing librarys are good enough to read the numbers the way they are presented and that you are experienced/determined enough to pull it off.
From what I found it would be easiest to do this in C# with the .NET 2.0 Framework - a language I’m not so familiar with. I don’t know how you would do this in detail but found a tutorial that points you in the right direction. The Asprise library for Optical Character Recognition is also available for this language.
If the absolute position of numbers dynamically vary as players are eliminated you need a way to locate them, which could be difficult. Hopefully this is not the case, so that you can manually input the number of players in the beginning - which would correspond to the positions of a set of bounding boxes for capture. (Ideally you would only grab these rectangles and not the entire screen.)
When the players take damage there is short animation that make the numbers unreadable as they increase, but this should not be a problem since you continually update and would eventually capture the numbers while they are still.
However, this seems to me like a huge endevour - especially if you are not familiar with programming. Considering that the difference between two numbers is something most people can do in their head I wouldn’t say that this feature is worth your time, unless you’re mostly in it for the challenge. For more questions regarding this I think StackOverflow or similar is a better forum. Your project isn’t really related to Twitch API, even though you intend to use it for Twitch.