Every time you run the keyword research, 1 credit is used up, regardless of the number of keyword results.
Depending on the plan you are on, you will get a set number of monthly keyword research credits. If you run out of credits, you have the option to wait until your credits are replenished on your monthly cut-off date or upgrade to a plan with a higher number of credits.
You can see your current credit usage in the top right of the Keyword Research page:
As seen in the example above, 20 credits have been used out of 100 available. The total available will vary depending on your plan.