v1.1 has now:

Maximum number of connections allowed for a user. The limits are enforced

separately for IMAP and POP3 connections, so you can move this setting

inside protocol {} to have separate settings for them. NOTE: The user names

are compared case-sensitively, so make sure your userdb returns usernames

always using the same casing so users can't bypass this limit!

#mail_max_user_connections = 10

Is 10 a good default?

Currently new connections just fail authentication with a "Maximum number of connections exceeded" error message. v2.0 hopefully will instead disconnect the oldest idling connection with IMAP.