Alot of his ideas were actually mainstream in western nations. He was considered THE European stateman along with Mussolini before the whole situation escalated with arms races.
Oswald Spengler released a book after WW1 called “the decline of the occident” which discusses in heavy detail the decline of the White man. His books were best sellers across all European countries and I believe the USA, although the man was a German.
These themes were popular in Europe and the USA as there was a very nihilistic attitude among the masses of decline, you have to remember people read Nietzsche and such themes.
So Mein Kampf was just another European books down the list of the recurring theme of “What to do with Western civilization now”?
You have to remember…Even if its controversial…That none of what is considered controversial today was controversial then: Antisemitism, nationalism, theories of Nordic superiority, decline of white race due to lack of demographics, rejection of mass industrialism and modernism, spiritual volkish concepts, etc…
As to the exact number of sales I am sure this was not so massive. After all Hitler was not Nietszche or Spengler. He was a politician not a writer. Surely after Nazi takeover, the book sales increases as the book was like “a bible next to every family’s bed” at that point and it was seen as proper to have a copy just to decorate.
But in terms of writing and just general quality, it was probably not considered very highly by Germans. But it was bought nonetheless as it reflected the themes of the time.