How Many Wall Plates Per Room?
Most rooms should have at least one wall plate that can provide at least a phone line and one network line. Larger rooms such as bedrooms, offices, living rooms, etc should have at least 2.
Step 1: Determine Furniture and Traffic Patterns
In some cases furniture will only fit in a room a certain way, a bed will only work in a bedroom on one particular wall for example. In most cases though there's some flexibility.
What I find works best is to try to identify traffic flow between fixed elements such as doors and windows. These will restrict the locations where you can place furniture. Sketch out your floor plan and mark the major paths people would flow through the room. For this tutorial I just grabbed a floor plan from 2dplan.com and drew arrows indicating the major flows of traffic through the rooms.
Notice how the traffic patterns split the rooms in 2 or 3 sections. Furniture will need to be laid out in a way that doesn't restrict the flow of traffic. The most convenient location for the jacks will be near certain furniture. Since the traffic patterns help guide furniture placement we can also use them to place wall jacks.
In the Master Bedroom for example we have 2 main flows. From the entrance to the window wall and from the window wall to the bath/dresser/porch doors.There are really only 2 ways the bed can be placed in that room. Either on the left wall as shown, or on the top wall. (Okay, 3 if you count in the corner between those two walls but I'm not a big fan of beds in corners.)
Step 2: Put a Wall Plate In Each Traffic Zone
The paths of traffic split up the room into different zones. In Bedroom 1 for example, there's one main traffic path from the entrance door to the patio door that splits the room in half diagonally. The bed is in the top half and the desk is in the lower half but it could easily be set up the other way around.
If we place one wall place on the top wall and one wall plate on the bottom wall we can easily hook up a phone on a nightstand next to the bed, a computer on the desk or a TV opposite the bed in all reasonable furniture placements without having to run wires across the flow of traffic (trip hazard) or without having to run long, unsightly patch cables.
Also consider most rooms will have a sitting/sleeping location, a viewing location (location of TV). The traffic patterns will normally split things up into workable sitting and viewing locations but make sure you can accommodate different sitting/tv locations with your wall plate locations.
Here is how I would run structured wiring and install wall plates in the sample floor plan to get the most functionality and versatility. Each orange dot represents where a wall plate would be installed.