Depends on what you're doing. Usually a prompter displays text on a 50/50 mirror directly in front of the lens so that the reader appears to be looking into the lens. On the other hand, if you have someone addressing an audience, or you're shooting a soap, you put a bunch of these in strategic locations so that they're in the actor's eyeline. For an executive or politician addressing an audience you might have one to the left, one to the right, and one on the main camera.

For a scene in bed you might put one on the ceiling.

Of equal importance is having the software to run it and a box to distribute the signal to all the screens.

You could also get showcard and a big fat marker and make cue cards.