Each competitor added to at least one keyword group uses one competitor credit. The number of keyword groups a competitor is tracking in does not affect competitor credit cost, though competitors that have been added to the Competitor Management page but not assigned to a keyword group will not cost any competitor credits.
For example, if you add competitor A, competitor B, and competitor C to keyword group 1, competitor B and C to keyword group 2, but do not assign competitor D to any keyword groups, the total competitor credit cost will be 3 (one for competitor A + one for competitor B + one for competitor C + zero for competitor D). Even though competitors A and B are added to two keyword groups, they still only cost one credit apiece. Competitor D will not cost anything, since it's not tracking in any keyword groups.
Each week, the Competitor Discovery and Competitor Discovery Trend feature will automatically discover the top 50 competitors for each keyword group and search engine. These competitors do not affect competitor credits until they are added to a keyword group.
There are 4 places you can visit to check for available competitor credits:
Competitor Credits in Account Detail page
Competitor Credits in Competitor Management Page
Competitor Credit in Competitor Discovery / Competitor Discovery Trend page