Have you got a lot of websites? Now there are chances that you don't have to monitor them all! It is because of the fact that Google has limited the number of websites registered in Google Webmaster. According to Google Groups, the number has been limited to 500.
However, if you're a partner, it's possible for the number to be raised, but it's a manual process, according to JohnMu.
At the moment the 500 site limit cannot be lifted for individual accounts. You might however want to take a look at the Webmaster Tools API, which allows you to verify sites and submit Sitemaps automatically.
At the moment, there isn't a way to increase the limit for individual users. Just out of curiosity, how many sites would you like to have verified there and what kind of information are you looking to get out of having so many sites verified with one account? he further said.