Ok, thanks for the answers. Thats what i wanted to know. So i would need a SSD if i want to run around 20 VMs right? I just wondered that a friend of me told me his server (lower specs than mine) can handle 21 VMs with no issues.
Second question: what virtulization is better for me? (running windows server 2012 vms) Xen or KVM?
I tried KVM without virtualizor and i only can handle like 10 vms (cpu is at 100% than).