Hi,
the answer for your question, is not that hard to calculate, however there is no 1 correct answer.
the footprint of 1 Vuser is mainly dependent of it's structure, amount of steps, data, amount of extra processing etc.
so my recommendation is to run 1 Vuser /10 Vuser and calculate:
the consumption of private byte it holds,
this could give a good estimation for the amount of memory you need.
the other limitation is CPU power,
I suggest to use the same method, however CPU is harder to anticipate (too factors can influence it)
more recommendations:
use physical LGs if possible, great CPU power is needed for big loads.
do not utilize more than 70-80% of CPU per one LG as it could obscure your testing
good luck!
Yoav