Supervisors have extra 'powers' than those of an 'ordinary' user - such as the ability to split & merge questions, warn (or block) users for their conduct etc. Supervisors are usually people who have extensive knowledge of a specific category - although there are also 'floating' supervisors who have no definitive category of their own.
Prospective 'candidates' are nominated by existing users - for their contributions, and conduct. For example - I wasn't even aware anyone was actually taking notice of all the answers I was posting to reptiles questions (my favourite subject) - until I had a note on my message board inviting me to become a supervisor ! After I recovered from the shock - I was more than happy to accept !
Sadly, no, you cannot. Another Supervisor might want to supervise that same category, so no 'rain checks' are allowed for categories. However, when you are old enough, you can co-supervise a category with another category supervisor for that same category you wanted to supervise.
The worst category to supervise on wiki answers is the technology category because of the accuracy of the answers expected.
Yes, you can Supervise more than one category.
Sometimes. If there are already enough supervisors in a given category, you may be able to supervise a sub-category or another category. There are also "floating" supervisors, who are generalists that have basically the same powers as category supervisors, but just help out wherever they're needed.
Yes, once you are a Supervisor, you can request to supervise any category you wish.
There are often multiple supervisors for a given category so that is not a problem.
The only thing a Category Supervisor has that a Floating Supervisor does not is something on their profile that says the category(ies) they supervise, the category supervisor badge, and their username on the category that they supervise. Floating Supervisors and Category Supervisors still have the same super powers and abilities - Category Supervisors just have the fame of supervising a category, albeit it takes more responsibility.
Generally just try and keep the category(s) as clean as possible. Generally you focus your time on these categories.
You can go to browse categories and click on which one you would like to supervise, then click "supervise this category" which will appear on the right side of the page.
There is not a set number. It is up to the discretion of the site.
If a category isn't supervised, then there isn't a Supervisor who is willing/able to supervise it.
Well, they can... but then they become Category Supervisors. There isn't really a reason, only that we have different names for the different kinds of supervisors... ones that like to supervise specific areas are Category Supervisors, and ones who like to float over the whole site and work in a lot of places are called Floating Supervisors.