Based on the number of machines, you will need a good performance to handle the load. (You don’t refer what OS’es or Services like Exchange, SQL, etc. you will install on them. This might prevent the scenarios below.
If this is for a home/testing lab I suggest:
1 drive for the OS
4 configured with RAID 0 (to give you the best performance).
If this is not for testing purposes and you need some reliability, I would buy another drive, and configure them this way:
2 RAID 1 for OS
4 RAID 0+1 for virtual machines.
There are other possible configurations, but for a generic approach I think this would do a good job.