It kind of seems you already asked that question. Of course you can just run a head count or a poll, but a figure from one of the few (the only?) hardware provider these days would at least give you an estimate. Of course I'm aware that there may be hundreds of active VC-4000 and APVS 1292 owners who don't subscribe to the multicart principle and would be missing from the sales figure.
Now are you asking out of curiosity, or to gauge interest in new products (software or hardware) before you invest time and money into those projects? I seem to remember you used to be one of the original developers, looking to get back into the system so it would make perfect sense to determine if it is worth your time to develop new games, if anyone is willing to play / buy those in the end.