Typically one full screen for your active window. The slave windows are really un-necessary, but do serve a convenience of being able to visually see a slave run off in a direction or, be rooted, killed, etc. A decent screen resolution being 1600x900 on high, and FPS limits at 60 for the main screen, meanwhile the other clients can have their FPS throttled down considerably, and if you get comfortable, turn down their detail setting to "Fair".

You will be primarily looking at the one screen anyway.

Personally, I only use the smaller client windows to verify an IWT action, and to verify any missed aggro... Else the entire team is usually visible from the main client window.