In case anyone cares: Thread:1437826
That is fine, the real reason I pointed you here was because I didn't feel like retyping everything. Regardless, this thread isn't that old. Once a thread reaches a month or two, then it becomes questionable (depending on the topic).
For quantity, are you talking about general terms such as "one", "a few", "many", etc. or are you talking about actual counts? If the latter, I am not sure using categories is the best solution. Perhaps if you could explain how you intend to use it?
As for rarity, it sounds to me like that would be a predefined list, would it not?
To address your actual question, you can implement point #2 by doing something like this.
<includeonly>[[Category:{{{artist|Infoboxes needing an artist}}}]]</includeonly>
This should not categorize the template itself yet it should place the page with the infobox into a category according to the artists name. If no artist is provided, it will place it into the default category "Infoboxes needing an artist".