Quick answer: MILLIONS!

[remember, there is a NNTP<-->EMAIL gateway involved, so this is "mirrored"
to comp.sys.hp.mpe, and any messages/threads started there are reflected to
the list]

As to "direct" participants, you might be able to find that out from the
Listserv software -- I know another e-mail reflector called "majordomo" that
generally allows people to query the server about the people subscribed to
the list(s) maintained by that program.  Try sending the message "help" to
the subscribe/unsubscribe/control address and see if there is something
similar [there may be, but it may also be disabled or "reserved for admin
users"]

> -----Original Message-----
> From: John Backus [mailto:[log in to unmask]]
>
> I participate in the Interex HP9000 Admin list ... One person emailed me:
>
>  The last I heard there are over 1700 members of this list,
> do you know what
> kind of mail overload you would see if it were open to
> analysis/discussion.
>
> I would like to say, 'yes, I do, the HP3000 list for example
> has over ???