(20 participants is a low number of participants)
BigBlueButton (centralized, open source)
I like BigBlueButton, that is Open Source, so you can find multiple providers, even local ones.
My beloved organizations use BigBlueButton for 80+ participants for their assemblies. It has a collaborative whiteboard, breakout rooms, and all bells and whistles I usually need.
BigBlueButton is usually faster even if people has low connection, since it's centralized server-side. If you have a good server, that's fast.
https://bigbluebutton.org/
https://demo.bigbluebutton.org/
https://en.wikipedia.org/wiki/BigBlueButton
Jitsi Meeting (peer to peer, open source)
https://meet.jit.si/
Or, Jitsi Meeting. Also Open Source. But Jitsi Meeting is peer to peer, so it's probably lightweight server-side, but it may be slower if participants have a slower connection.
Anyway, you will not find difficulties in finding Jitsi Meeting websites that work perfectly for 15-20 participants.
All my mentioned solutions are Free Software, so are available in multiple ways:
- just as gratis courtesy service
- as "self-host", so, you can just install that (at no cost) on your own server by yourself.
- as paid "as a Service" (so, somebody else takes care of everything, and it just works, and it gives to you reliability, etc.)
- as paid "On Premise" support (so, somebody else install for you the software in your infrastructure, if you already have a server that you want to use, and you just pay for assistance from an human being)
Note that instead Google Meet can only be delivered "as a service", but it's a non-open-source service. Same note for Zoom and Microsoft Teams.