running 12 on the following setup...
i-2700k OC'd to 4.7ghz (the limiting factor)
*this cpu is also obsolete, the new version is the 3770k from intel
32 gb ram
evga 560ti 1GB superclocked(no need for anything more expensive than this for 12 bots seeing as how you're being limited by your cpu, not your gpu) if you want to run more than 12 bots, get a graphics card with 2gb memory.
mobo is asus p8z77, though you can buy any mobo that has socket compatible with your processor, ie 1155, 2011, etc
750w psu
SSD or HDD - I got one small SSD and run the clients with isboxer. If you don't want to pay for isboxer, then you will need at least a 256gb drive. Cheaper to get an HDD but they will load the clients slower. If you're gonna buy everything else that this system needs, then you may as well spring for the SSD too in my opinion.
Liquid cooling for cpu - also not completely necessary but if you're going to be running a cpu OC'd at full load 24/7 then it's not a bad investment.
This rig can run 12 but they will start to lag if i try to use the computer for anything else, like watch a movie or surf the web even. If you want to run 12 and also use the machine for other basic computer stuff, then this won't cut it. At least it doesn't for me.
cpu - $300
gpu - $220
ram - $180
mobo - $170
psu - $110
cdrom - $20
tower - $50
ssd - $200
liquid cooler - $80
Total - $1330
Black Friday's around the corner, I bet you can get all this stuff on sale. Problem is now you also gotta buy 12 D3's plus farming gear for 12 bots :|