Credits are calculated with this formula:
Keyword x Engine x Device x Country x City/Zip
So if you are tracking a keyword ('blue widgets') in both Google and Yahoo, just desktop, just in National US-en...that would count as two keyword credits.
Let's say you wish to track your 'blue widgets' keyword in just Google, but in 5 different country locations. That would cost 5 keyword credits. The same goes if you wished to track a keyword in say 10 different zip codes or cities, that would be 10 keyword credits.
If you wanted to track 1 keyword in both mobile and desktop in just Google, that would be 2 credits.
You can view your current keyword credit limit as well as usage in your account settings page.
What if I track the same keyword in multiple projects?
Note: If you track the same keyword/location combo in multiple projects, only 1 of the combinations will be counted against your total. (up to 4 competitors). So if you had 4 websites tracking all the same 'blue widgets' keyword in the city 'Portland, OR' that would only count as 1 credit, not 4.