17 days sounds funny

(grammar questions I couldn't answer)

I was doing a CAT lesson with my students, and in the course of the discussion the following exchange took place.

Student: We have 17 vacation days at my company.
Me: 17?  That's a funny number?  Why 17?
Student: What?
Me: It's just that 17 days sounds funny.  Why 17?  Why not a nice round number?

After the conversation, when we were writing up expressions on the board, the expression:
17 days sounds funny got highlighted.

The students asked if the expression shouldn't be:
17 days sounds a funny number

As I usually do, I answered first instinctively based on my native speaker intuition, and then afterwards tried to think of the rule.
Instinctively I said no, "17 days sounds a funny number"  just sounds wrong.
But what was the rule?

Put on the spot, I came up with some sort of explanation of how copular verbs like "seems" "feels" and "sounds" are followed by adjectives, and not noun phrases.
Was that right? I don't know, but it seemed to satisfy the students.

But there were more questions.

"17 days is in the plural" another student said, "so the verb shouldn't have the 3rd person singular s.  It should be  '17 days sound funny'."

Again, my native speaker intuition was telling me that 17 days sound funny was wrong, and 17 days sounds funny was right.  But why?

Again, put on the spot I came up with some sort of explanation that "17 days" in this sentence wasn't referring to a quantity of days, but to an expression that I thought sounded funny.  So it took the singular verb.
The students seemed satisfied with this, but again I'm not 100% sure I gave them the right explanation.