The construction fire protection standard answers this with three numbers, and most sites get the first one right and forget the other two.
Area and travel distance. A fire extinguisher rated not less than 2A must be provided for each 3,000 square feet of the protected building area, or major fraction thereof, and travel distance from any point of the protected area to the nearest extinguisher may not exceed 100 feet (29 CFR 1926.150(c)(1)(i)). Both conditions apply: a large open floor may satisfy the area count and still have a corner more than 100 feet from the nearest unit.
Floors. One or more extinguishers rated not less than 2A must be provided on each floor, and in multistory buildings at least one must be located adjacent to the stairway (29 CFR 1926.150(c)(1)(iv)).
Flammables. An extinguisher rated not less than 10B must be within 50 feet of wherever more than 5 gallons of flammable or combustible liquids or 5 pounds of flammable gas are being used on the jobsite; integral fuel tanks of motor vehicles do not count (29 CFR 1926.150(c)(1)(vi)). A roofing kettle, a generator fueling station, or a cutting-torch setup each trips this one.
Upkeep. Firefighting equipment must be conspicuously located, periodically inspected, and maintained in operating condition; defective equipment is replaced immediately (29 CFR 1926.150(a)(3), 1926.150(a)(4)). Access to all available firefighting equipment must be maintained at all times (29 CFR 1926.150(a)(2)).