While giving a lecture on Paul Elder in 2004, I was asked this question. It’s every bibliographer’s nightmare, of course: failing to record a title.
I am confident that the current online checklist records at least 99% of Elder’s output; the uncertainty concerns that last one percent. All the earliest records of the bookstore were destroyed in the 1906 earthquake and fire. Many of Elder’s books are quite rare: there are several titles in my collection where—despite all my years of looking—the copy I own is the only one I’ve ever seen. That doesn’t necessarily make them valuable, just rare. There are also plenty of titles I do not own, though not for lack of searching.
Recently the rate of new discoveries has slowed to one every couple of years, although in 2009 I found a catalog with twelve new titles in the Impression Classics series. So I am reluctant to guess how many titles I have yet to find.
A list of checklist addenda & corrigenda can be found here.