What are my options for running a larger scale web conferences (up to a thousand endpoints perhaps, handful of them speaking) with free software and regular web

What are my options for running a larger scale web conferences (up to a thousand endpoints perhaps, handful of them speaking) with free software and regular web browsers involved? I guess past a certain point it's more like streaming than group calls. What have larger f/oss events such as FOSDEM used during the pandemic and what did they have to change to get it to scale?