What kind of settings are you currently running? What type of performance are you getting out of your game clients? Do you feel that you could double the number of game clients?

Where are you taking 10 characters? Any situation you'd need 10 characters for (that I can think of) is going to be resource intensive (e.g. lots of players, lots of spell effects, etc), so if you're struggling to push something like 60 FPS on your main and 30FPS on your slaves at this time, then I highly doubt you're going to be able to add five additional game clients to the mix unless you're willing to drop settings down into the floor (i.e. very low).

This is just my guess.