"Two mangoes"

Translation: Maembe mawili

[DJdMbP]

Does the "b" in "mbili" change to a "w" no matter what class of noun it is grouped in with?

[Ernst557459]

Well, not all noun classes, only the ones that indicate the plural of a noun (for obvious reasons :)
Otherwise, yes, with the exception of the N/N class. This is the list:

II) Watu wawili - Two people

IV) Miti miwili - Two trees

VI) Maembe mawili - Two mangoes

VIII) Viti viwili - Two chairs

X) Nyumba mbili - Two houses

Please check this thread about the adjective concords of the N/N class: https://forum.duolingo.com/comment/30738085