Without wanting to turn this into an English lesson;1-I'm not neglecting Kabuto's words. Many don't believe in his power not "most" as you stated. Furthermore even Obito knew he was the strongest of his generation.
The word 'many' when used with a plural verb (such as doubted, which means 'to doubt'):
n. (used with a pl. verb)
1. A large indefinite number: A good many of the workers had the flu.
2. The majority of the people; the masses: "The many fail, the one succeeds"
2. We will never agree.