A very large number?
I will now attempt to give you a more helpful answer that might indirectly answer your question. In the other HN thread out there today "Those making $1,000+/month on side projects – what did you make?" one of the answers is from Wes Bos who made a book and series of videos on Sublime Text editor. He claims, and I have no reason to believe he would lie, he has grossed $80k in 3 months selling the book at $45 and the book and videos at $60.
Is the JavaScript engineering market large enough to make money off of tutorials/ebooks? I would say if the content is good, the writing is well done, and the marketing is fresh, you will do exceptionally well. If Sublime Text Editor can bring in that kind of money, can you imagine JavaScript? Of course, the flip side of that coin is market saturation.
Another way to answer this would be, if your biggest concern is potential size of the market, you should be golden. It is huge.