The recently revealed French and Spanish titles of upcoming film Star Wars: The Last Jedi seem to have settled the debate about the title's meaning.
Possible spoilers ahead for Star Wars: The Last Jedi.
LucasFilm has tweeted the French and Spanish title cards for Star Wars: The Last Jedi (via The Independent), which seem to imply that the title refers to multiple Jedi rather than a singular "Last Jedi."
In French, the title is "Les Deniers Jedi," while in Spanish it is "Los Ultimos Jedi." Both titles use grammar that refers to a plural subject - in this case "Jedi" - rather than a singular subject. For the non-English majors out there, "Jedi" is one of several words which in English is spelled the same when referring to one Jedi and multiple Jedi.
While this doesn't explain what that means specifically, it seems to likely refer at least to Luke Skywalker and Rey, who sought Skywalker out to return his lightsaber (and perhaps seek Jedi training) at the end of Star Wars: Force Awakens.
Star Wars: The Last Jedi is scheduled for a December 15 theatrical release.