Why does WireGuard only support 20 connections
I supports only 20 instances. Each instance can support a multitude of separate connections to various endpoints. What's your reason to configure that many instances?
I do not know if this limit is in WireGuard or specific to the OPNsense implementation. I never needed more than 2 or three instances, though.
Its just a limitation of the early days where WG was unstable. I cant imagine where you need more than 10. Only if you use for every client a dedicated instance (for whatever reason)
I go from site to site and have many branches. Can you tell me how to remove this restriction?
But you need just one instance and N endpoints to connect N branch offices to a HQ in a star topology ...
I have more than 100 branches with different IP network segments. How do I set them?
All as endpoints with a single instance. You do not need a tunnel network! Just allowed networks and that's it.
Can this achieve interworking site?
Sure :)