Changing default number of machines users can add to a domain

The Active Directory attribute you need to change is mS-DS-MachineAccountQuota which is a property of the domain object. 

– Start ADSI Edit (start/run/adsiedit.msc)
– Expand out the Domain node, right click on DC=<yourdomain>,DC=com and select properties
– Scan down to ms-DS-MachineAccountQuota
– Modify the value as appropriate, or clear the value to remove the limit entirely.