Star Wars Episode VIII has a subtitle. “The Last Jedi” evokes a sense of time running out, of a last stand. It is ominous.
Whether this refers to Luke Skywalker, or to Rey, is unclear. During the events of Star Wars Episode VI: Return of the Jedi Skywalker became the last Jedi, according to Master Yoda.
The word Jedi is plural however. It is a mass noun, or a non-count noun. This means a word that represents both the individual, and the group, in contemporary English.
A discussion on Stack Exchange has some examples of the use of the word to mean single and plural.
For more information, Empire Online published an interview with Star Wars Episode VIII director Rian Johnson.