Theoretically no, but in practice when you get more than a couple of hundred you may encounter performance problems. Also when you get into large numbers of fora then maintenance can become a big problem.
No - only Admins can create a forum.
Members can subscribe to a forum which would give this functionality.
Not sure what you mean by "automatically send emails". Members can send emails to groups (if the correct permissions/settings have been done).
If by "master moderator" you mean Admin then yes.
Within phpBB - no, but there may be limitations applied by your host and/or mail server.
Creating forums is an administrator permission. Moderators by default have no administrator permissions, but you could still assign that one permission to those (or even users) - that would result in letting them access to the ACP and its module of managing forums plus all modules where administrators have access in general.
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