Probably a couple of thousand active users, using a broad definition but not counting those who only use emulators to play retro games.
Unfortunately I'm one of these users who only uses an emulator at the moment. Having used an, A600 and A1200 in the past (sold the last one in Dec '98).
However, with the release of OS4 I'm planning to get an A1 - if/when OS4 is release........ ;-)
It should be this core of ex-users Aminga Inc target when the new OS is released - should give a steady boost to initial sales.