Pretty much as many as you need. There is a hard limit, but it is quite high and wil lbe removed in the next Hyper-V generation (which is due in a week or two).
At the end, what you ask makes no sense because the limit is - IF you are limited - more one of performance, which does actually depend on how the CPU's are USED. if your physical CPU hits it's performance limit, then the number of virtual CPU's you can support is useless. You wont get more performance out.
You are a LOT more likely to be limtid by RAM or - most of the time - IO, than by a number like this which has no real meaning.
if you still want it ;) You are limited to 12 virtual processors per logical processor when all g host is is R2 Sp1 Sp1 and the guests run windows 7, otehrwise the limit is 8.
That would put your limit to 64, or 96, depending on the clients. But again - this is a useless number most of the time.