The ecommerce software market is changing and growing so rapidly and so are the stores running them (setup and taken down all the time), so Im not sure if there is data out there that can tell you accurate numbers on actual used carts.  I just opened up shop with BigCommerce.com, a hosted solution.  They claim 7500+ stores are run by them.  Many others will also tell you a general number of customers.  So maybe you can take the top used carts and identify that way?