It's a bit of a "how long is a piece of string" question. I do recall though somebody mentioned CPU was about 1-1.5% per instance on a machine not dissimilar to yours.
Then of course it depends upon the pathces you are loading, if you use the ramsave feature and of course how many VSTi's your host supports - e.g. CubaseSX3 has 64.
Then of course it depends upon the pathces you are loading, if you use the ramsave feature and of course how many VSTi's your host supports - e.g. CubaseSX3 has 64.