Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I don't know of any particular limit, although I wouldn't suggest having too many. What number are you talking about? I personally have 9 right now, without issues. The bigger issue is how large each mailbox gets. I would recommend never having more than about 1000 messages in a single mailbox, and keeping the inboxes as small as possible. I subscribe to the zero inbox approach of trying to remove items from my inbox as soon as they are completed/read, etc, but Mail is also known to have problems with very large mailboxes, and the Inbox is particularly susceptible due to how often it's accessed.

jW
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.