You most likely want
https://antmedia.io/
Ant Media WebRTC streaming library. It's the back-end for most adult streaming sites and alternative livestreaming sites. Cheapest option by FAR if done on budget hosts, especially if bandwidth is free or cheap (ex. Hetzner Cloud US gives 10TB free).
Some other options:
Free:
https://jitsi.org/jitsi-meet/ Free/Open Source, built for video calls but also works for livestreaming*
https://livekit.io/ Free SDK, similar complexity to Jitsi but tailored specifically for live streaming
https://www.ovenmediaengine.com/olk Roll-Your-Own auto-scaling live-streaming solution
...
API as a Service:
https://www.100ms.live/ Super low-latency livestreaming API with premium pricing
https://castr.io/ All-In-One, ok pricing
https://www.api.stream/ Good, pay as you go pricing
https://liveapi.com/pricing/ Good, pay-as-you-go pricing
https://www.simplelive.co/livestream One click embedded live stream start-up, pricing seems too good to be true
...
* Example repo which used Jitsi for livestreaming to thousands: https://gitlab.com/guardianproject-ops/jitsi-aws-deployment
...
All of the options will run ~$100 to $3000/month for streaming 100 hours a month to 1000 users, with APIs costing the most and bandwidth being the main cost of self-hosted solutions.
Using AWS, Azure, or Google Cloud anywhere in your stack can easily 2-3x your costs above the high bound.