My objective was to look at virtual machine management systems (such as SCVMM and Virtual Center) and to consider how many VMs they could manage. More here
The potential items that you can run into are many.
1) Overhead on the management system just because of the number of VMs.
2) Limits to the number of VMs on a single host.
3) Host storage
And this is without actually powering on a single VM.