In the case of VPS, the question does not make sense since the VPS takes all the resources of the host.
In the case of VMs running inside a VPS or deployed with custom buildout script on bare metal, there is no definitive answer.
Yet, a reasonable overcommitting limit for each VM is:
It is then possible to adapt the memory and CPUs used by each VM depending on their needs. This is controlled by qemu's maxcpu and maxmem parameters.
A reasonable overcommitting ratio depends on the type of usage. A 400% overcommitting may be appropriate to maximize system load, at the expense of transient performance.