Obviously, each instance of the VM will require a license.
MATLAB Named User Licenses are per-user per-host. With my Named User license, in theory I could run an indefinite number of VMs on a host that I own or lease, as long as they are all for my use (not, for example, handling work on behalf of a team.) With a "Standard" (sometimes called Commercial or Professional) license, I could in theory be doing that on two different physical hosts simultaneously.
So, for at least for Named User licenses, there is no per-VM requirement in the licensing.
... However...
In practice, Named User licenses are keyed to Media Access Control (MAC) addresses, and in practice each VM needs a distinct MAC in order to be able to use IP. If you are using a Named User license for your own work, and you can find a way to share a MAC between the host itself and the VM, then you are within your license rights to do so.
If this is VMs running on something like azure, then the licensing situation is a bit different, and I am not sure how licensing works for that situation.
