According to http://support.microsoft.com/kb/251335/EN-US, the default quota for adding computers by a user to a Domain is 10. If you attempt to add more than 10, you will likely get the error message:
"Your computer could not be joined to the domain. You have exceeded the maximum number of computer accounts you are allowed to create in this domain. Contact your system administrator to have this limit reset or increased."
To resolve the problem, you can run adsiedit.msc on the domain controller and from there change the DOMAIN attribute "ms-DS-MachineAccountQuota" from 10 to something larger like 500.