password - The Security to the Party [Part 18]

Blah, bah, blah, party, blah, blah, blah, spaghetti.

First guest arrives, guard says "3", guest says "0", and is admitted.
Second guest arrives, guard says "4", guest says "1", and is admitted.
Third guest arrives, guard says "8", guest says "2", and is admitted.

Fourth guest arrives, guard says "9", guest says "3", and is admitted.
Fifth guest arrives, guard says "10", guest says "4", and guard says "Umm, I don't know, come back later."
Sixth guest arrives, guard says "11", guest says "5" and is dissolved by acid.
Seventh guest arrive, guard says "18".... what should the guest answer?

Here's the series: https://puzzling.stackexchange.com/questions/tagged/party-security

Hint:

I may need to edit the question later

I won't accept an answer without an explanation of why it is right.

Answer

My reasoning

The response is the accepted response to the given party security puzzle - #3 had the answer 0, #4 is currently unknown.

My answer

We seem to be asked for the answer to this very challenge, which presumably means I can pick any number. I'll go with 12.