As of ver 0.3, parallels-desktop-plugin provisions as many VMs as Jenkins master requests at a time. This results in multiple VMs running simultaneously despite their configuration, which:
a) Will be slower than expected
b) Might result in something like described in JENKINS-40685 (it happened exactly when Parallels Desktop was starting up two VMs with the total memory greater than the host memory – the hardcoded timeout wasn't enough)
The suggestion is to implement a pre-provisioning check that the total resources required by the multiple VMs to be provisioned are not greater that what the host has. The check could be optional (a checkbox in Cloud configuration).