Since it's a short incorrect sentence, it's not obvious what the the speaker meant, and one can't point to just a single error in its construction.
As noted in the comment, "quarantine" is a transitive verb, so it can be passivized. It is also a noun. Here are two examples of correct use of the word that may be what the speaker intended:
1 How many days is he quarantined for?
This uses the verb "quarantine" in passive form, meaning "He is quarantined. I want to know how long the quarantine must last."
2 How many days is the quarantine?
This uses "quarantine" as a noun, and asks how long it must last.
Per comment by the OP, it seems this is what the sentence meant:
3 "How many days is 'quarantined'"?
that is, having heard the word "quarantined", someone jokingly wanted to know how many days that implied. Note that for that use, he should have used quotation marks around the word "quarantined".