In the Trailer from Spider-Man Far From Home Mysterio says, that the MCU is Earth-616, but in an official book it was stated, that the MCU is Earth-199999. The seems like a conflict, but it can actually be resolved. Mysterio don't says, that the MCU is universe 616, he says that it is dimension 616 and there is a difference between dimension and universe. For example Earth-616 has its own dark dimension and Earth-199999 also. That means Dimension 616 and Dimension 833 mentioned by Mysterio are both just dimensions inside Earth-199999 like the dark dimension and the quantum realm.