A) Using the phrase "a certain number" when that number is one is a pretty contrived example.

2) That's not a very standard definition of "case". Many linguists don't even admit that case exists in English outside of pronouns, but I don't know anyone who defines "case" as "nouns or pronouns".

Þ) When using such phrases as "a/the number of" or the like one must be very careful to determine whether one is talking about the number or the thing being numbered. "The number of insects on the wall has been increasing over the past few days" vs. "a number of insects have been crawling on the wall over the past few days.