I don't believe that there is a maximum number of attachments, however you do
want to be conscience of the fact that there are limits defined on the
overall size of a users' mailbox and if a message would exceed that capacity
if the server accepted it, it would then be rejected. In other words, you
can't send a 600 MB message (attachments are included in determining the size
of a message) to someone who has a 500 MB mailbox.
These size limits range from a couple 100 MBs to sizes that meet the needs
of customers who pay the bill.