Feature request: degree of parallelism in testing on LXD platforms

If I need to test on many distributions with LXD there is no way to say 'please do not create more than "N" containers'. Let me clarify with an example, lets say I want to test MicroK8s on 42 distributions. Launching 42 lxc containers at the same time and testing on them is not practical. There is no way to express that I want at most 3 lxc containers (mapped to 3 distributions) simultaneously running.

Thanks