NEC section 210.52(A)(1) (2002 edition) gives the minimum requirements for outlets in the living areas of a home: "Receptacles shall be installed so that no point measured horizontally along the floor line in any wall space is more than 1.8 m (6 ft) from a receptacle outlet." paragraph (2) defines a wall as any unbroken space 2 ft or more in width, including space measured around corners, and free-standing counters and bars, but excluding sliding panels. This is the minimum requirement. I have never heard a complaint about having too many outlets.
There is no minimum spacing. There is also no minimum distance from a doorway. Use good judgment to avoid a tripping hazard or damage to cords and equipment.