Typically speaking in high density designs we use a single AP to cover 1500 Sq foot. In medium density it can be every 5000sq foot.
For HD layout that is an AP every 44 feet with clear line of site and around 15dBi Tx power. The reason is if you are doing an array of AP's you have self healing built in if you have an AP failure. The others have power envelope to ramp up. It's the same with audio:
Say you have distributed sound throughout. A speaker every 44 foot radius. You have a total amount of gain structure to 21dB. You run everything at 15dB though. If you have a speaker fail you have 6dB of headroom before you hit your power constraint with the next closest speakers to make up for the loss.
But you have to take into account anything that acts as a baffle: Wall construction type, elevators, metal stairways, brick, metal impregnated glass, etc.