Hard to say, but remember that the majority of cachers simply do not attempt anything other than traditional caches, ignoring puzzles and even multicaches.
Now consider that you have to have a smartphone and install the Wherigo app, or have a very specific GARMIN GPSr, then download the cartridge before you can attempt the Wherigo....
Now on top of that they aren't very common, the builder is still in beta mode and apparently not at all user friendly. I try for all Wherigo's I can and have only found 29 in the last few years.
Blue -
