Aaah ok, so you can use the following (for everyone who has the same problem)
20 results
https://api.twitch.tv/helix/subscriptions?broadcaster_id=$broadcasterID
100 results
https://api.twitch.tv/helix/subscriptions?broadcaster_id=$broadcasterID&first=100
Specifically a user
https://api.twitch.tv/helix/subscriptions?broadcaster_id=$broadcasterID&user_id=$user_twitchID
But what is the parameter After?
https://api.twitch.tv/helix/subscriptions?broadcaster_id=$broadcasterID&after=100
https://api.twitch.tv/helix/subscriptions?broadcaster_id=$broadcasterID&after=1
don’t work. But why?