Warning: SPOILERS for No Time To Die.
No Time To Die never reveals James Bond's (Daniel Craig) new 00- number and it becomes a running joke throughout the film. Early in No Time To Die, Bond meets Nomi (Lashana Lynch), the new 007. Even though Bond swears that being 007 is "just a number," the presence of his replacement provokes a sense of rivalry in James when he ends his retirement and goes back into active duty at MI6.
Of course, James Bond is synonymous with being 007 and he's always been the only 007 in the franchise. No Time To Die breaks tradition by someone else taking his job and number. MI6's other Double 0 agents have been referenced and seen throughout the previous Bond movies. The most famous other Double O agent was 006 AKA Alec Trevelyan (Sean Bean), the villain in GoldenEye. Otherwise, all of the Double O agents at the time were assembled for a meeting in Thunderball, while Bond has encountered or avenged the deaths of some of his peers in other films, such as 002 AKA Bill Fairbanks, who was killed off-screen in The Man With The Golden Gun.
No Time To Die treats the fact that Bond gets a new 00- number playfully, as the basis of an amusing running joke at Nomi's expense. But No Time To Die never revealing Bond's new number is likely to service to the audience who still (and will always) regard Bond as 007 even if someone else happens to carry that famous number. Meanwhile, Nomi becomes the audience surrogate by repeatedly asking, "What number?" and being denied an answer. This implicitly tells the audience that James' new 00 number doesn't matter because, at least for a time, Nomi and James Bond are both 007. No Time To Die not actually saying Bond's new numerical designation keeps the audience from being even momentarily distracted by having to think about Bond having a new number so they can continue to focus on the film.
The scene in M's office when Nomi discovers James Bond is back as a 00- agent also positions Nomi as an outsider in M's inner circle, even though she is 007 now. Bond's return reminded Nomi of the long history between James, M, Eve Moneypenny (Naomie Harris), and Q (Ben Whishaw). Indeed, everyone keeps their mouth shut as Nomi keeps changing the subject back to Bond's new number because they're all in on the joke that Bond is essentially stealing "007" back from Nomi without ever lifting a finger or saying a word. Going into No Time To Die's explosive ending, Nomi realizes who 007 really is as well.
The interplay between James Bond and Nomi is one of the most interesting and understated aspects of No Time To Die. They engage in a game of one-upmanship throughout the film but they also gain mutual respect for one another. After Dr. Madeleine Swann (Léa Seydoux) and Mathilde (Lisa-Dorah Sonnet)—Madeleine and James' young daughter—is kidnapped by Safin (Rami Malek), Nomi surprisingly asks M for special permission to reinstate Bond as 007 before they assault Safin's island fortress. By that point, Nomi dropped her competitiveness and revealed that she does admire Bond. After all, Nomi took over the most storied code number in MI6 history, so she understood the magnitude of the legacy she inherited even if she couldn't help teasing Bond about being his replacement.
It's not clear if it was ever James' plan to essentially trick Nomi into surrendering the 007 number. Bond did seem genuinely touched by the gesture and he was grateful that Nomi volunteered to join him in attacking Safin's base to stop Project Heracles and rescue Madeleine and Mathilde. However, M's reaction was curious as he seemed to have been expecting Nomi's move, so perhaps MI6's leader suspected that James was playing with his young replacement and angling to get his number back, which ultimately worked. Regardless, No Time To Die had to return James' proper Double 0 number to him so Bond could fittingly die saving the world as 007. By any other number, whether or not it was revealed, No Time To Die beautifully ended with the reaffirmation of James Bond as the one, true 007.