I am only using one agent on the main build server, and we have an extra five-pack of concurrent build licenses - but still the number of concurrent builds never exceeds two.
Is it bound to the number of physical cores?
The server is VM on Azure
Processor Intel® Xeon® Platinum 8171M CPU @ 2.60GHz, 2095 Mhz, 2 Core(s), 4 Logical Processor(s)
and it is insistant on running only two concurrent builds.
The old server was a VM on a regular server.
Processor Intel® Xeon® CPU E5-2643 v4 @ 3.40GHz, 3392 Mhz, 4 Core(s), 4 Logical Processor(s)
and it happily ran six concurrent builds.
Why the change in behaviour?