SerpBook is based on keyword/domain combinations (ie. 50 for Basic, 100 for Essential).
If you subscribe to a 50 keyword package, you would be able to track 50 url/keyword combinations. Google, Yahoo, and Bing updates are all included in the package.
You may also update your keywords on-demand, meaning whenever you wish (additional to the normal bi-hourly updates you currently receive).
Our packages can be found here.