Idea about Registering rare computers.

I was talking to someone earlier today about how many Altair 8800 systems have survived up to the present, and obviously all that we could do is make a guess. Does anyone know? Also, has anyone thought about creating a web site for users to add system that they have to it for tracking purposes. For instance, if you have an Altair, you go to the site and add your system to the list of Altair owners. It might list the serial numbers, model, owner, etc. This idea could be used for many old rare systems. Basically, a registry of owners and their systems. You obviously wouldn't need to include something like the Commodore 64 or Apple IIe but at least the more early computers, like the Apple I for instance. That way, once a system is listed, collectors can somewhat keep track of them informally. How many Apple 1 or PDP-8 systems are left?