You'll need the URL of a specific Slack channel.
It looks like this https://app.slack.com/client/YOUR_TEAM_ID/YOUR_CHANEL/YOUR_CHANEL.
You can choose to maximize the number of members to extract by setting the maximum results.
By default it is set to 1000 and max to 5000.
If you want/need to extract more people, you should segment your search into smaller batch.
To use this setting with the API, you can pass max_results to the input_parameters field.