Start off by heading here and taking a look at which package suits you most. You can choose between a wide variety of options that will easily suit your needs.
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. This means 50 domains with 1 keyword, or 50 keywords on 1 domain.
There is a discount when you subscribe on an semi-annual (6 months) or annual (12 months) billing cycle.