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: Marge and Homer.
Marge and Homer will reflect as 2 respondents in your billing.
You can add Marge and Homer in Standup 1, Standup 2, Standup 3, etc. - your account will be charged only for group = plan, including 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 the trial ends, you open Billing and see no "package" for 2 respondents. In this case, you will need a bigger package, for 3.