I don't think the website is any indication of how many characters each series will get. While the desktop website might show one thing, the mobile site is entirely different.
For example, while the Mario series appears to be completely filled on the desktop website, the mobile website setup makes it seem as if there's space for one more character.
Basically, I don't think any particular theory on slots or the website are going to bear any fruit unless somebody finds a way to get into placeholders on the website and what not, or finds a way to datamine something.