Hi thunder11, I am the author of the ISPConfig tutorial you refer to above. I am no expert in load but I can tell you the physical spec I used for the tutorial, which is also the current production spec.
The physical host machine :
System : PowerEdge R410
CPU : Intel Xeon E5645 2.40GHz
RAM : 16 GB 1333 MHz
HDD : 1 TB RAID1 (2x1TB SATAII)
All my ISPConfig servers run on virtual machines.
VM panel : 1 vCPU, 2GB RAM, 20GB HDD
VM db1 : 1 vCPU, 2GB RAM, 100GB HDD, hosts 148 user databases
VM web1 : 1 vCPU, 2GB RAM, 200GB HDD, hosts about 40 sites
VM web2 : 1 vCPU, 2GB RAM, 100GB HDD, hosts about 20 sites
VM mail1 : 1 vCPU, 2GB RAM, 50GB HDD
VM ns1 : 1 vCPU, 500MB RAM, 3GB HDD
VM ns2 : 1 vCPU, 500MB RAM, 3GB HDD
Unfortunately I have no load information available, however we do host some websites with several thousand impressions a day.
In addition the same host machine hosts other virtual machines not part of the ISPConfig setup.
Our websites have to my understanding no problem with response times
If you want to use virtual servers, I recommend you go for a host machine with 6 or more cores each core running 2 threads - as I understand it the number of virtual servers you can run before starting to swap CPU resources between virtual server is number of cores multiplied by number of threads / per core subtracted one (for the host).
The above physical host machine have 6 cores each with 2 threads, which gives me a maximum of 6x2-1 = 11 virtual machines before the CPU swapping will hamper the performance.