Do you have more then 500 websites and wan’t to use Google Webmaster tool for all of them ? According to the latest thread at Google Groups Google webmaster tool have a limit of Maximum 500 Websites and you can’t add more then 500 sites into one account.
However, You are allowed to have one more Google webmaster Account to manage them !!
According to JohnMu, If you’re affected, you can hop on over to Google Groups and ask for some help.
Source Search Engine Round Table.