There are a few podcasts that have discussed this. Typically it is done via support for existing OSS, offered to companies who need to be able to depend on full knowledge & stability of that code. A couple of examples:
1) Redhat
2) "some guy" from one of the podcasts I can't think of at the moment who forks Ruby and keeps a stable, supported version for his corporate clients, while dealing with patches & upgrades on his own to his fork
In both of these cases, you can see that is isn't actually "how many users", but "how many corporate users who need a service that keeps software X completely stable and managed"
So I think from what you've said that it would be a good thing to look into. You may want to contact the guys from http://www.tropicalmba.com/ for a few pointers - this is right up their ally and they are very willing to discuss
1) Redhat
2) "some guy" from one of the podcasts I can't think of at the moment who forks Ruby and keeps a stable, supported version for his corporate clients, while dealing with patches & upgrades on his own to his fork
In both of these cases, you can see that is isn't actually "how many users", but "how many corporate users who need a service that keeps software X completely stable and managed"
So I think from what you've said that it would be a good thing to look into. You may want to contact the guys from http://www.tropicalmba.com/ for a few pointers - this is right up their ally and they are very willing to discuss