virtualbox.org

I have a Windows 7 x64 Host with 32GB RAM and an 8 core processor.
I have a number of Windows 7 x64 Guests. All are working satisfactorily but I am interested in expanding the number of guests . I currently normally do not run more than two guests at the same time.
Where would I look for performance limiters in this sort of arrangement?

Are you asking how to limit performance, or asking what would limit performance? I'll assume the latter.

Performance will be limited by whatever shared resource runs out first. If the VMs need lots of CPU then you only have N-2 free cores (assuming you leave 2 for the host). If the VMs are doing lots of disk or network I/O then of course your hardware has a finite I/O bandwidth. Likewise with RAM, graphics performance etc.

p.s. Do you really have an 8 core processor, or do you actually have a 4 core with hyperthreads? There's a difference, or people wouldn't pay a premium for the real thing. In fact why not just have a single core and 32 threads?

I was thinking about what would limit performance as you surmised.
You are also correct I have a 4 core with threading by INTEL.
Essentially I am considering running CPU intensive work on a separate virtual machine from the one/s that generating the load. For example a localhost server of xAMP kind in one machine and generating the load in the others depending on how many I can get to run. Based on your analysis it would seem the limit might be Host+xAMP+ 2 others.
Would you agree?

On a 4 core host where CPU intensive work is planned, I wouldn't have much hope to run more than 3 VMs (assuming each VM has the same profile and the host can get by with one core). However, only testing can tell you for sure.

Most apps these days are limited by I/O, not CPU. In fact most apps spend most of their real time waiting for user input, but I assume that doesn't apply to you.

Thanks for the comments.