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 poll only for two: Misha and Masha.
Misha and Masha will reflect as 2 respondents in your billing.
Misha and Masha can be added in Standup 1, Standup 2, Standup 3 etc. - your account will be charged only for group = plan, which includes these respondents.
Respondents can't be bought separately, but you can purchase different "groups", starting from 3 respondents.
Example: You added only Misha and Masha, but when trial ends you open Billing and see that there is no "package" for 2 respondents. In this case, you will need bigger package, for 3.