Respondent = each separate Slack user, added in a Respondents and Requests section of a report.
We count respondents in general, not per each report.
Example: You have 3 Slack users in your team, but you decided to run a standup only for two: Marge and Homer.
Marge and Homer will reflect as 2 respondents in your billing.
Marge and Homer can take part in several reports. In your usage tab, they will still be displayed as 2 users.
Respondents can't be bought separately, but you can purchase different "sets"
Example: You added only Marge and Homer, but when the trial ends, you open Billing and see no "set" for 2 respondents. In this case, you will need a bigger set