Storage requirements for running virtual machines

I am looking to run virtual machines using NFS and iSCSI protocols. I have installed Openfiler in a VM appropriately sized with 2 vCPUs and 4 GB RAM and 20GB disk

I would be adding a second virtual disk to this virtual machine for which I am looking at hard disk capacity between 1-2 TBs. This would be provisioned from a hard disk connected to my desktop using eSATA. Question here is how much capacity, which type of enclosure (Antec or Transcend)?

I can connect a 1 TB HDD using eSATA external enclosure although I am not sure if that would be appropriate to run all VMs (aroun 20-30 of them).

I have 2 ESXi hosts (physical servers) installed on a USB drive.

Please let me know if you require further information.