How complete is VD16? There are many sixteenth-century German printed editions that have entirely disappeared, of course. But how close is the database getting to covering everything that still exists in some form?
Completion is still a ways off yet. One type of evidence for that is how easy or difficult it is to discover undescribed editions. New incunables are still discovered fairly often, but it's not easy to find them - I've never managed it, despite several years of looking. On the other hand, intensive work with the card catalog in just about any large collection of sixteenth-century books, or a long look at the collection in a library off the beaten path, would often turn up editions that hadn't been cataloged in VD16.
Another kind of evidence is the number of editions being added. Since around this time last year, VD16 has added around 757 new records, putting it around 104,000 total records and over 122,500 titles. I believe the only sigla to which new titles are being added are ZV and XL (for German titles printed outside of Germany). If I'm reading things correctly, VD16 has added XL 134-159 over the last year, and ZV 29126-29866. By June of this year, the additions to VD16 will probably bump into ZV 30000, a somewhat amusing test entry meant to illustrate the underlying record format.