There are a few different ways one can add more than the default of 10 Windows systems to Active Directory. The methods are listed below:

1. Be a member of the Account Operators group or Domain Admins group
   
2. Explicitly add the "Add Computer Objects" to the default Computers OU for the domain being added to
   
3. Change the "ms-DS-machineAccountQuota " domain attribute using adsiedit.msc

In the lase case, you MUST be a Domain Admin in order to modify the attribute in question.  You can refer to the following article for more information on how to set the value correctly:

http://books.google.com/books?id=Ga1nLk_xqcUC&pg=PA377&lpg=PA377&dq=active+directory+add+computer+maximum&source=bl&ots=MIb-xmBTPf&sig=UIcopurNSYtMltzPMpSgz8NBz8w&hl=en&ei=7p6qSsa5GIHQsQP5-JSWBQ&sa=X&oi=book_result&ct=result&resnum=8#v=onepage&q=active%20directory%20add%20computer%20maximum&f=false