As per my knowledge there doed not exist any hard and fast rules about adding site to analytics. But to know more you can check the Google analytics guideline.
Google Analytics will track as many websites as you own! You're limited only by the 5 million pageview limit per month - but even that is removed if you're also an active AdWords user. More info visit:http://support.google.com/googleanalytics/bin/answer.py?hl=en&answer=55542
Although you can add as many as want but it is difficult track well all of them. So i would say that having in groups or categoriesed is better in different accounts of analytics.
You can add as many blog as you owns. Coz GA have limited you to track 5 million pageview per month as I think it almost equal to unlimited for me as got just 3-4k average Pageview per month. So buddy just add all your blog and let GA track them. Wish you all a very Happy and Prosperous NEW YEAR 2012
As far as I know there is no limite. At one time we were managing over 32 analytics accounts and that was fine. How many are you using at the moment?
As i know there is no limit for Google Analytics.. I have an account having more than 350 projects in it and all are working fine.. few of projects having more than 50 thousand visits per months... I went through the google support url http://support.google.com/googleanalytics/bin/answer.py?hl=en&answer=55542 but in case of my account i don't think that there is any limitation of "5 million pageview limit per month" per account..