Snapcast has a webapp and a native android client. Although I'm not sure how well it handles many, many clients. In theory, if all on the same WiFi they should all play in sync like a silent disco (at least for those not using Bluetooth headphones where the playback latency is too high/not available).
Web radios handle many clients. The first problem could be if the Wi-Fi hot spot can handle that many clients. The second one is that web radios and their protocols usually don't care if two clients are not in sync. They are usually in different places, maybe different continents.
I'm self hosting a web radio for my LAN at home. I set it up years ago, I'm not there so I can't check the details but I think it is: Icecast2 on an ARM small server with DeeFuzzer (sp?) to send my mp3s to the Icecast2 server. MPV or VLC to play music on my Linux laptop and Transistor from F-Droid (I believe)
They handle many clients but they absolutely do not play in sync. That's never a requirement for them and I'm not aware of any web radio protocol supporting that feature. Web radio is not the right solution for a silent disco type situation where you can at least guarantee everyone is relatively local.
