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If you have 20GB of memory, you cannot use full. Because hypervisor will reserve some memory for OS layer. Rest you can use. And for your question, if you have 20GB and you have a flavor of 20GB memory, yes only one person can use it, i mean you can deploy only one virtual machine using that flavor, unless you over committ the memory factor.

When existing instance is not using the resource, the hypervisor will manage to get the resource and distribute to the instance who need it. This idea is happening through a virtio corridor and called Ballooning

There is a way in openstack for Autoscaling the resources. Read the project here