Set memory and multiprocessing preferences by choosing Edit > Preferences > Memory (Windows) or After Effects > Preferences > Memory (Mac OS).
As you modify settings in the Memory & Multiprocessing dialog box, After Effects dynamically updates helpful text in the dialog box that reports how it will allocate and use memory and CPUs.
The RAM Reserved For Other Applications preference is relevant whether or not Render Multiple Frames Simultaneously is selected. The settings in the After Effects Multiprocessing category are relevant only if Render Multiple Frames Simultaneously is selected.
Todd Kopriva provides information about optimum memory and processor settings on the Adobe website.