MSExchangeTeam blogged about objects limits in Active Directory.
We frequently get this question in many newsgroups and forums - what's the maximum number of members you can add to a Distribution group? The member attribute of groups - both Distribution and Security groups, is a multi-valued attribute. So the answer is more about how many values can a multi-valued Active Directory attribute hold.
Many of you may remember the recommendation of 5000 values in a multi-valued attribute in Windows 2000, and the fact that the limitation no longer exists in subsequent versions. So what's the actual limit? Or is there a limit at all?
Active Directory Maximum Limits info: http://technet.microsoft.com/en-us/library/cc756101.aspx
Read more at source: http://msexchangeteam.com/archive/2009/02/19/450729.aspx