A number of / the number of is an idiomatic pair. The rule does not apply to other cases. The verb does not depend on profits; the subject is 'portion': A/The portion is....
The only one I can think of, yes. The point is that 'the number of' is singular (noun 'number') but 'a number of' is an idiomatic quantifier meaning 'several/some/many'. If you can find other phrasal quantifiers with an embedded noun, please let me know.