I am working on an infrastructure refresh for the company I work for and I need to do capacity planning. My boss felt somewhat burned by VMs he purchased from a hosting company not living up to the performance level expected, so any capacity numbers for KVM VMs that I can use as a basis for my calculations are appreciated.
It depends on how busy they will be and how much ram you plan on allocating to them. For a basic setup, I would consider 1-2 gb for each vm, so you should be able to run 10-20 if they aren't heavily cpu bound.
The art of capacity-planning is not an exact science - it does depend on several inputs - chief amongst which are:
The target you should be looking at is roughly 75% CPU & Memory Usage for all VM instances.
KVM does have fairly good memory management and as such you can overallocate [balloon] memory that would be made available by the host.
Red-Hat/Cent OS have a similar feature as VMWare ESX where KVM can page in/page out memory segments shared by different OS's - hence you can squeeze even more VMs.
Thus - lets assume you are running 11.04 desktop for each VM instance. The majority of the work is users using Firefox & LibreOffice - taking up, say half of the required memory (Ubuntu minimum is a recommended 1GB), but little in the way of CPU usage. I would expect (wet finger in the air) for a 24GB host to be able to run 32 VM's. On Red-Hat/Cent OS - you perhaps could increase this to 48 VM's.
Again - not an exact science, but not knowing what the chief areas (1 to 6 above) are, just an educated guess.