I am not sure if I can help.
Typically, initially all students have access to a new quiz. However, if you want, you can also include a password for the quiz which restricts access to it. You are correct that there is only one password allowed. But why would you want more than one, realistically?
Well, maybe you want to restrict access to this quiz for one particular (or even several) students. If so, you can set up a "user override" and change several of the characteristics of a quiz, by user. You can even "duplicate" an override, which makes it quicker to create a bunch of these "overrides." This is how I would do it.
Quite honestly, I have never used a password on quizzes in the 6+ years that I have been using Moodle. I have, however, created (too) many overrides, typically where I want to change the due date for a quiz, or allow a student with disabilities a little more time for a quiz. If you are coming to Moodle from other LMSs, like Blackboard, which does not provide "user overrides", then passwords might be needed on a duplicated quiz (very awkward, but this is the solution). Moodle's "user override" feature is much better than other LMSs (BB, D2L, Canvas), and I love it.
Yes, many instructors have many different kinds of scenarios, so I do not doubt at all your specific needs, I am just wondering along with you about the best way to handle your specific needs.
Attached is an screen shot of the "user overrides" feature in Moodle, if you have not already seen this.