WordPress.org

This is a plugin for wpmu/wpmu+byddypress powered websites, where network administrators can limit the number of blogs a user can signup. It is pretty simple and adds an option to NetworkAdmin->Dashboard->network settings page, where you can limit the number of blogs. No additional frills required.

What It does

It adds an option to the Network settings page when you are logged in as site admin of the wpmu site(or wpmu+buddyppress site),Look at the bottom of options page,and You will see a text box like this asking for number of blogs allowed per user.If you set it to zero(which is the default),It will not restrict the blog registration then.