Let's take a look at each of the report types you can run in InternalLinking.
Top 1000 pages
InternalLinking gets the top 1,000 pages for a site from Search Console, ordered by clicks and then impressions. InternalLinking will then request the server rendered HTML for each of these pages and look for instances of the text you want to link from to the target URL.
This report costs 5 credits each time it runs, as it is the report that requests the most amount of pages to look for link target text.
Google Search
This report uses the Google search results to find candidate pages to build internal links from. It will do the following search site:yoursite.com 'query' and get up to the top 20 results for each query you are wanting to build links from.
InternalLinking will then check the HTML of those pages returned for usage of the query you want to build links from.
This report costs 2 credits each time it runs.
GSC Pages
This report queries Search Console via the API for all the pages it has displayed in the search results for the queries you are wanting to internally link from.
InternalLinking will then request the HTML for each of those pages and check for the query.
It's likely that this will return the smallest sample of pages, hence why it only costs 1 credit each time this report runs.