Let's just say you end up running 10 virtual machines on 1 computer, think about the requirements for that. For each VM you will be running lets say 2 instances of WoW, both running no addons and on lowest graphical settings botting would need about 500mb of ram and a bit of cpu grunt to power it, then times that by 10 VM's would be about 5gb ram and a decent to high end cpu i would reccommend something with more than 4 cores.
My recommendation for this build based on 10 VM's Running 2 clients each (20 total wow instances)
6-8gb of ddr3 low latency ram
3.8 ghz + quad core cpu
some good cooling for both system and cpu
and maybe a medium end graphics card something with say 1-2gb of ram should be plenty
it would net you around $1,000 I would go and research you some prices for the parts but that really comes down to your preference. Hope i've helped somewhat