My subjective guess is that around 1000 likely still exist. But if you wanted to do a slightly more scientific study you could visit the 914-6 Registry and count all the cars registered there. Their webpage is
http://www.stazak.com/914reg/.
That would tell you how many 914-6 were registered, and thus existing as of a few years ago. Of course more 914-6 will exist than the 914-6s registered on that site but you will know
at minimum how many 914-6 exist. Other regional or national pca webpages can help augment that data with their own members lists, which typically indicate what sort of cars their membership is driving.
But my basic matrix (totally unsubstantiated SWAG) is to take the 914-6 registry number and muliply it by about 1.5 and that should give you a rough idea of the number of cars out there.
Douglas