There is no limit,  but obviously the cost will be very  high if you have
thousands of lists. They  must all be polled every N  minutes, and when a
local user  sends a LIST  command he will get  a listing of  thousands of
lists (when he probably meant to see the network-wide ones only), and you
can be sure this  listing will take time to prepare  and send back. There
are many  other instances  where a  lot of  CPU time  will be  burnt, for
instance every  night the server  will want to  send updates for  any non
local list which  has changed since last  time. Well you may  only have 5
non-local lists, but it will still have to scan the N thousands before it
knows that.
 
  Eric