I reduced the amount of subs to 1 per page and tried it on my own channel, as I only have 70 subs.
On the 30th call, I get back: {"data":[],"pagination":{}}
URL is: https://api.twitch.tv/helix/subscriptions?broadcaster_id=37749713&first=1&after=eyJiIjp7IkN1cnNvciI6IjIwMjAtMDgtMDdUMDI6MTQ6MTZaIn0sImEiOnsiQ3Vyc29yIjoiMjAyMC0wOC0wN1QwMjoxNDoxNloifX0
{“data”:[{“broadcaster_id”:“37749713”,“broadcaster_name”:“Metalus69”,“gifter_id”:"",“gifter_name”:"",“is_gift”:false,“plan_name”:“Channel Subscription (metalus69)”,“tier”:“1000”,“user_id”:“267199061”,“user_name”:“Ip_gaming1”}],“pagination”:{“cursor”:“eyJiIjp7IkN1cnNvciI6IjIwMjAtMDgtMDdUMjM6NDM6MTVaIn0sImEiOnsiQ3Vyc29yIjoiMjAyMC0wOC0wN1QyMzo0MzoxNVoifX0”}}
Fetching next resultset with pagination ID: eyJiIjp7IkN1cnNvciI6IjIwMjAtMDgtMDdUMjM6NDM6MTVaIn0sImEiOnsiQ3Vyc29yIjoiMjAyMC0wOC0wN1QyMzo0MzoxNVoifX0
URL is: https://api.twitch.tv/helix/subscriptions?broadcaster_id=37749713&first=1&after=eyJiIjp7IkN1cnNvciI6IjIwMjAtMDgtMDdUMjM6NDM6MTVaIn0sImEiOnsiQ3Vyc29yIjoiMjAyMC0wOC0wN1QyMzo0MzoxNVoifX0
{“data”:[{“broadcaster_id”:“37749713”,“broadcaster_name”:“Metalus69”,“gifter_id”:"",“gifter_name”:"",“is_gift”:false,“plan_name”:“Channel Subscription (metalus69)”,“tier”:“1000”,“user_id”:“38162465”,“user_name”:“Safire224”}],“pagination”:{“cursor”:“eyJiIjp7IkN1cnNvciI6IjIwMjAtMDgtMDdUMjM6NTM6MjdaIn0sImEiOnsiQ3Vyc29yIjoiMjAyMC0wOC0wN1QyMzo1MzoyN1oifX0”}}
Fetching next resultset with pagination ID: eyJiIjp7IkN1cnNvciI6IjIwMjAtMDgtMDdUMjM6NTM6MjdaIn0sImEiOnsiQ3Vyc29yIjoiMjAyMC0wOC0wN1QyMzo1MzoyN1oifX0
URL is: https://api.twitch.tv/helix/subscriptions?broadcaster_id=37749713&first=1&after=eyJiIjp7IkN1cnNvciI6IjIwMjAtMDgtMDdUMjM6NTM6MjdaIn0sImEiOnsiQ3Vyc29yIjoiMjAyMC0wOC0wN1QyMzo1MzoyN1oifX0
{“data”:[],“pagination”:{}}
Reached end of results, no more to fetch.