Interesting Facts

• Maximum number of objects in Active Directory: A little less than 2.15 billion
• Maximum number of SIDs in in a domain: About 1 billion
• Maximum number of group memberships for Security Principals: 1015*This is for Security groups. Each Security group you're a member of results in its SID being added to your access token at logon.

Note: For Windows 2000 Active Directory environments, the recommended maximum number of members in a group is 5,000. This recommendation is based on the number of concurrent atomic changes that can be committed in a single database transaction.

http://technet.microsoft.com/en-us/library/cc756101.aspx