There's a collaboration going on between several universities and museums that is attempting to find and catalog and create a digital archive of all American samplers. Now that's quite a project.
There is a finite number estimated between15,000 and 20,000. The Sampler Consortium has located more than 10,000 samplers in American collections though some of these may not be American in origin. Details of this research is here, though it looks like the website hasn't been updated in several months.
They are still looking for information on samplers and family names associated with the District of Columbia.
To find the details of the this collaborative project being done in part by the University of Delaware, go here: sampler project
Imagine being able to look at nearly every schoolgirl sampler stitched in America.