Less than 4 walls
This assumes typical drywall. For ideal deployments, imagine a signal from an AP to a user not going through more than 4 walls.
Don't count on cross-floor bleeding
Due to ceiling heights and material used between the floors of multi-floor structures, very little signal propagates between floors.
Distance of 50-70 feet
Walls and height of AP matter a lot, but a typical AP can support a diameter of 50-70 feet and still provide good signal.
Square feet 3,000 to 5,000 feet
Design where each WiFi AP on average services 3,000 to 5,000 feet of office space with usual office ceiling height.
Number of client connections
Try to limit the number of client WiFi connections to each AP to 30-50 connections/users. There are many factors here including the number of streams the AP supports, typical client type (mobile or laptop), types of data being accessed (rich media like video versus web sites), etc. When dealing with high density applications such as many smaller cubicles, auditoriums, etc. then an emphasis on client connections and using load balancing features becomes more important.
If you follow the above, you'll generally end up with one AP per classroom or one AP for every two classrooms. If one AP per every two classrooms, definitely use higher end APs to support more users and throughput.
Interference and noise floor
All the above don't take into account the interference from neighboring APs or the RF ambient noise floor in a given location.
Bottom line, it's all about dB loss and more loss is worse, following are some examples:
- Elevator shaft: 30 dB, concrete: 12 dB, brick wall: 10 dB, thick door: 6 dB
- Drywall: 3 dB, thick window: 3 dB, bookshelf: 2 dB, thin door: 2 dB, cubicle: 1 dB, thin window: 1 dB
Use the right antenna for the job. Omnis (omni directional) are the normal pattern, but specific needs can benefit tremendously from the right antenna.
Location, location, location
Down on a desk is bad, on a file cabinet is worse, on a vertical wall without external antennas is not ideal, above a ceiling tile right side up is bad. You still need to know about RF energy dispersal patterns and plan accordingly. Location does matter in terms of not picking the bad spots.