The five user limitation is due to MSDE only handling five concurrent
connections, and a single user may have more than one connection
depending on what they are doing.
The load on the machine hosting the database may also limit the number
of users who can work comfortably--e.g. watching a DVD, or some other
CPU intensive activity, on the host will also slow MSDE and BCM
clients' response times.
Theoretically, if your BCM server machine had a full version of SQL
Server and sufficient CPUs you could potentially have an unlimited
number of BCM clients.
That said, I expect that there is an absolute number of BCM clients
with which the database locking mechanisms used by the BCM code would
begin to throttle the number of operations possible on the database.
Since BCM is not intended to support a large number of users, it was
not designed for performance with a large number of clients.
It would be interesting to get some numbers on the absolute number of
clients possible when MSDE and hardware are not the limiting factor.