When you write "free software" it seems you really mean free-of-charge services.
I can recommend to use Free Software (a.k.a. Open Source), which means software that is both free-of-charge and free-from-control - i.e. software that is bound by ethically responsible ownership rules.
Most famous Free Software tool for video conferencing is Jitsi Meet.
It is a tool, not a service. Multiple providers offer services using this same tool.
You can see a list of community-run Jitsi Meet services here.
Less famous, but equally good, is the Free Software tool BigBlueButton. I am unaware if there is a list somewhere of services using BBB - one that I am aware of is Senfcall - free of charge, just requires you send them an email requesting access (and then you can create video rooms where you control access - it is not that all your participants need to get special access).
Yet another, more lightweight, video conferencing tool is backend Mediasoup using e.g. frontend multiparty-meeting. One service using that combo is letsmeet.no.
Even more lightweight - and my personal favorite -
is backend Janus
and e.g. frontend Jangouts.
You are welcome to try use my service using that combo,
All of these services are - as far as I am aware - free for use however long you like, and should work with 20+ users if your bandwidth can keep up with that (otherwise you may need to limit amount of users with video enabled).