We can't really answer such questions as they are lot's parameters involved that will completely change the result.

The type of application that you are testing, HTTP or HTTPS, RIA application like Flex, Oracle Forms, Siebel etc or simple web application etc..

The complexity of your script can also impact the number of VU's that will be handled by a single LG.

Usually the best is to test with your real script and run it with a ramp up load policy. Then monitor the memory/CPU utilization of your Load Generator and then you will get a good idea of how many users your machine can generate.

Expect something between 500-750 VU's if it's a simple HTTP application otherwise much less.