What are the requirements for server performance in librenms（hardware）， if I have more than 200 network devices, thousands of monitoring items, what kind of server is needed?
Thats a loaded question. With thousands of devices I hope they all support snmp and I would do distributed polling. Separate DB and web server.
Have you looked here ? https://docs.librenms.org/Support/Example-Hardware-Setup/
And
https://docs.librenms.org/Extensions/Distributed-Poller/