At this point, I know of three - one for PC (which is the version I am most familiar with), and two newer versions for PS4 and PS Vita (which was released digitally on August 15, 2017).
I vow to be truthful, so I hope my answer won't impede my credibility. BUT, I have actually never played UNDERTALE all the way through. I have, however, seen it played all the way through in multiple different styles by some of my favorite YouTube gamers.
No way! I only listed a few of the most obvious ones that relate to Schell's discussion of mechanics. I'm sure there are plenty more that can be talked about, and not to mention even more from Schell's text that I didn't include.
The short answer is, space. I had written a collection of journal entries on each of the game mechanics that are looked at in Schell's book, but as it came time to add them to the site, I realized they were much too long. So, I elected to include only the ones that felt the most pertinent.
I was introduced to the text by a professor at my university. He is the one whose classes I combined my work from in order to create this site!
Yes! I'm sure that more practiced web designers will be able to tell rather quickly, but I choose not to see that as a bad thing. You have to start somewhere, and this is the place I chose to begin! I can only go up from here.
I did this site as part of a team...of me, myself, and I.
No, but seriously, the project was on an individual basis, and apart from the assistance of my professor, I did it all on my own! (And if I can do it, anyone can do it).