Back when Lucasfilm revealed the title of Star Wars Episode VIII, it's been debated over the exact meaning. Specifically if it's singular or plural since the word Jedi is like sheep and can mean both. Thankfully the official German title has revealed all. The title is plural.
The official title, spotted by IGN Germany, is Die Letzten Jedi. If it meant a single Jedi Knight, the title would be Der Letzte Jedi. This means we don't have to get frustrated trying to figure out if it means Luke or Rey. It's probably both, at the very least.
Engadget Spain has also reported that the Spanish translation also indicates that the title means multiple Jedi. IGN has pointed out that a now-deleted tweet from Brazil seemed to indicate the title referred to a single male Jedi, but this German news seems to be all proper and official.