CPU and memory overcommit (oversubscribe) support
Is XCP-NG support CPU and memory RAM over commit?
Short answer: CPU yes. Memory not directly.
Could you point out the doc/reference/info about it?
Xen is scheduling for you, so as long one VM doesn't have more vCPU than physical core available on one machine, you can do whatever you like.
last time I did power up a lot of VMs, if I remember it, Xen didnt allow it because I ran out of vCPU,
Am I wrong maybe?
So no need to do anything right, just turn it on, it will work?
Yes, there's no problem to have in total more vCPUs used than cores.
It's just that you can't have one VM using more vCPUs than cores on the machine. There's nothing to do, Xen handles this for you.
What to do with RAM, how we can oversubscribe it?
Short answer: you can't.
Long answer: you can "cheat" with dynamic memory.
But in general, I would avoid it.