How many database users/admins should be created for a website?
You should at least not be using root for your application. The rest completely depends on how the application was developed. It sounds like you wrote it yourself, so my opinion is based on that.

My suggestion is to use two accounts, as you said one for write access (to the CMS database only) and one with read-only access (again, only for your CMS database). That way, if you have any kind of vulnerability with, for instance, the display portion of your application, you can somewhat potentially limit the damage an attacker can do.

You could do it with one user for the whole CMS; most applications you could download like WordPress and so on do it this way to make installation easier and because some hosts don't allow more than one user account. If you have the means to do so, I prefer to isolate roles as much as possible.

The reasons to not use root are many, but basically you want to give the user the minimal set of permissions needed; if an attacker gains access through your root account they can create new users, delete data, change configuration variables, and much, much more. If they gain access through a limited account that only has access to read and write to your CMS database, they can't create accounts, they can't delete other databases, and they can't insert malicious data in to other databases. They might mess with your CMS database, but it's the principle of limiting the damage at that point.