The number of VMs is directly limited by the storage capacity and performance requirements measured in GB (Gigabytes) and IOPS (Input Output Operations Per Second). For starters you need fast disks.
http://en.wikipedia.org/wiki/IOPS#Examples
- 7200RPM SATA drives – ~90 IOPS
- 15kRPM Serial Attached SCSI drives – ~180 IOPS
- Simple SSD – ~400 IOPS
- ioDrive, a PCI-Express card with Flash – >80,000 IOPS
How to “see” IOPS usage on ESX or vCenter…
http://communities.vmware.com/thread/273268
As an example, Windows XP 32 bit VM would need 10 to 20 IOPS and 10 to 20 GB Capacity.
Windows 7 – 64 bit would pretty much double that. A high performance server VM could easily need 2000 IOPS.
http://blogs.vmware.com/performance/2010/05/exchange-2010-disk-io-on-vsphere.html
And – make sure your product roadmap shows how many IOPS just ONE ESX host can drive…
VMware: VROOM!: 100,000 I/O Operations Per Second, One ESX Host
To demonstrate the scalability of the ESX I/O stack, we decided to see if ESX could sustain 100,000 IOPS.
http://blogs.vmware.com/performance/2008/05/100000-io-opera.html