A whole one! Atnas, you're dropping in standard, I have to say.

Chances are that RMXP:PKE is different to the bogstandard version, but battles are thus made:

First of all, you need to set up the monsters, and what is in each group. This is done in the Database under the headings: Monster and Monster Group.

To set up random enocunters, right click your map on the left hand menu of the map building screen, select Map Properties, and add monster groups to the listbox (Encounters) on the right of the pop up menu (just double click the space). The higher the encounter number is, the more random encounters there are, I believe.

To set up your own specific battle, make an event and choose "Enemy Encounter" from the third page of events.