Doctor Strange in the Multiverse of Madness has confirmed that "616" is the designated number for the MCU Prime universe.
Christine 2: Our universe is 838. And we've designated yours 616.
Strange: Oh. You guys sure must know a lot about the Multiverse. You got someone going around naming realities.
Marvel producer Nate Moore, during interviews publicizing the release of Spider-Man: No Way Home, called the universe that the films of the MCU takes place in “the MCU 616 universe” but this doesn’t appear to be any sort of official statement.
"You will not see T'Challa in the MCU 616 Universe. We couldn't do it.
I will say that when Chad [Boseman] passed, it was a real conversation
that we had with [Ryan] Coogler about 'what do we do'? And, it was a
fast conversation. It wasn't weeks, it was minutes of 'we have to
figure out how to move this franchise on without that character'. I
think we all feel that so much of T'Challa in the MCU, not in comics,
is tied to Chadwick's performance, is what he brought to that role
both on and offscreen, I would argue."