I've never thought of supporting 300000 queued builds, as OneDev optimizes for self-hosted market which rarely has such volume. With clusters and agents, OneDev should be able to support a quite large number, but I never tested the limit.

How easy could it be, to setup such a test, to detect the limits of the system?

Should not be difficult. You may submit build requests via restful api:

https://code.onedev.io/~help/api/io.onedev.server.rest.resource.JobRunResource

Note that OneDev will cancel previous build if new build with same job and commit is submitted. In that case, you may create a param for the build, and for each build, use a different param.