I have started watching Altered Carbon on Netflix. I have read the books long time ago. Why is Kovacs the last Envoy?
This is an invention of the Netflix series entirely. It stems from the change of "Envoy" from being the name of the Protectorate Peacekeeping force that is sent out to pacify rebellions and other such things (as it was in the book) to being the name of Quellcrist Falconer's specific rebellion (generally called Quellists in the book).
Takeshi Kovacs is, in the Netflix series, called "The Last Envoy" because Quell's followers are all considered permanently dead. Kovacs was caught at some point later and sentenced to storage until he was brought back in the first episode.
In Episode 7, it is revealed
Everyone else was taken out by a viral weapon that destroyed their stacks, except Kovacs, Reileen, and Quell herself. Quell and Reileen were on a shuttle that exploded shortly after.
It is of course also revealed in the course of the series that
Reileen survived and possibly Quell's stack did as well, which means Kovacs isn't the Last Envoy in fact, although according to the historical record it is still true.
In the books, Kovacs wasn't even "The Last Quellist," as it was a philosophy that had many adherents and many survivors of the actual rebellion, instead of a weird tiny revolutionary movement living in the woods that somehow had skills nobody else has seen since.