I've been looking through the max help files and online but still haven't found where the switch is to limit MR rendering to using only a certain number of CPU's. I like the speed of using all 4 but it's hard to get anything else done.
Also consider this program. I use it for older games and programs that go crazy with multi-core CPUs. http://www.majorgeeks.com/WinLauncherXP_d870.html You launch a program from this little tool, and have the options already set for which cores to use. :D
If you're using Windows, you can do Ctrl+Alt+Delete, go to task mananger processes, find the rendering process, right-click it and go to "Set affinity" - uncheck the CPUs you don't want it to use. If you're doing that to free up resources so you can still use your computer, I'd recommend instead of doing that to Set…