there are millions of potential amigas out there using aros or some varient of uae lets keep going :-D
seriously i think the 10 to 15k might be fairly accurate but i also think it is irrelavent.
number of machines isnt as important to me as number of users and that to me is were aros and uae become important. when it becomes possible to use aros full time there is the potential for alternitive os fans to switch. not everyone loves linux and many people dont like windows.