There are no practical limits to the number of objects stored in Active Directory. The Active Directory database has been tested for up to 40?million objects. Performance tests show logon performance for a single LDAP client to be the same with 10,000 objects, 100,000 objects, and 1?million objects — that is, the directory service does not slow measurably when the size of the database increases.
In a mixed-mode environment in which backup domain controllers are running Windows?NT?4.0, the recommended limit for the number of security principal objects per domain is 40,000 (the sum of users, groups, and computers). This limit is based on the Windows?NT?4.0 SAM database storage capacity.