This is a nice article on Active Directory Limits
Summary
Max Objects – 231 minus 255 or ~2.15 Billion or 2,147,483,394
Max SIDs – 230 or ~1 billion or 1,073,741,824
Max Group Membership for Security Principal – 1015 groups
Max FQDN Length – 64 characters
Max File Name Length – 260 characters [1]
Max OU Name Length – 64 characters
Max Group Policies applied – 999
Max Number of Directory Operations per LDAP Transaction – 5000[2]
(Recommended) Max Number of Domains – 800 (Windows 2000), 1200 (Windows Server 2003 FFL-2)
(Recommended) Max Number of Domain Controllers in a Domain – 1200[3]
[1] This is a bit tricky as this limitation is in the Win32 subsystem, not in NTFS which has something like a 64k limit. You can get around this limit if the tools you use use unicode path format with \\?\
[2] Wow, I have never been even close to that… I expect you would be close to if not over bumping the Max LDAP Packet size of 10MB (default)
[3] If over 800 DCs and using ADI-DNS, see KB267855.
