well im running 10 (didint try more), on the site it says:
With any given Intel processor, you can run 1 client per physical core your processor has.
With newer 3rd, 4th, 5th, and 6th generation Intel Core i5 and Core i7 processors, you can run up to 2 clients per physical core depending on the core clock.
With any Athlon or newer series AMD processor, you can run 1 client per physical core.
Note that the AMD Bulldozer 8350 and higher have 4 actual real cores. You're not guaranteed to be able to run 8 clients even though those processors advertise that they are octa-core.
You need 2 gigabytes of RAM, plus an additional 1 gigabyte of RAM for each client you want to multibox.
So it depends on how powerfull your pc is.