This might be what you are looking for:
Jitsi is a set of open-source projects that allows you to build and deploy secure videoconferencing solutions.
The source code is here.
You can use the browser as the client (multiplatform), and there is an SDK for Android and iOS.
The are no fixed limits (your resources will define that). No privacy information. The traffic is not end-to-end encrypted, just between the server and the clients. If you set your own instance, it would be up to you to comply with GDPR. If instead of auditing the source you would like to ask them about possible privacy issues, this would be the place to do it.
It is already packaged for Debian and Ubuntu. You also have instructions on how to set up up your own instance.