I've just noticed lately that when I "resort" my wish list to show the books in order according to author's name, I have fewer books than if the books are sorted by other criteria.

For example, I have 194 books on my WL as the books appear alphabetically, but when I resort them according to author's last name, I have 179.    Where did 22 of my books go?

And if I start adding books to my WL with them sorted according to author, will I be able to add more 22 books? 

Okay. . .I just tried to add more than 6 books and the system does not allow me to.  So that part of the question is answered. 

But why does the number change when I resort according to Author, I wonder?

If you have books that do not have an author listed, they disappear when you sort by author.  The book is still on your WL, it just doesn't show in that view.

Nope, you won't be able to add more just by changing how they are sorted. The books are still there on your Wish List, they just aren't viewable on the option you have selected at that time because they have no Author listed.

You can fix the issue by finding the books that don't have an author.  Click on the title, then on "more options" and choose "edit book data."   There will be a delay of several days to a couple of weeks before the change shows up, as it has to go through an approval process.