What is the least I can watch and still follow Doomsday?
Eleven titles in release order, and every one of them changes what the multiverse is rather than mentioning it. The old Avengers end, the TVA opens the branches, two universes touch for the first time, an incursion gets named out loud, the mutants arrive, a new team gets assembled, and First Steps closes on Doom. Left out on purpose: the Fox X-Men films, which Deadpool & Wolverine explains faster than they explain themselves, and Quantumania, which builds up to a villain this film is no longer named after. Everything skipped here has a summary on the list — read those and you are caught up.
Countdown to Avengers: Doomsday