General rule of thumb is 400 sq ft per ton of cooling. A ton is 12,000 BTU/hr, so you'd need 4 of those units. While you can buy larger units, you'd be better off with something like 4 12's as opposed to a pair of 24's, due to the fact that a couple larger units will have trouble throwing the air far enough to give even temperatures. 12's will normally run on a 120V circuit, while the larger ones will require a 240V circuit, which can cost more to add than the unit itself costs.
I think you'd find a single 12 won't do much for you, but you could start out with a couple 12's if you didn't want to go all in at once. If I was installing 2 12's, I'd attempt to locate them at quarter points along the 48 ft wall if possible. IOW, put one unit about 12 ft in from the end wall at each end of the building.
While this wouldn't be enough to hold the shop at 72* on a 90* day, it will be enough to make a big dent in the humidity level as well as lowering the temperature to a tolerable level. Humidity is the main factor in a shop being uncomfortable, and its surprising how comfortable even 80* can be when the humidity is low.
One other factor is the type machines you're running, and whether you're evaporating a lot of coolant in the cutting process. For example, if you run a single 15hp VMC at full load, all the electricity that goes into the machine ultimately ends up as heat, either sensible in the form of raising the air temperature, or latent in the form of evaporating coolant and raising the humidity in the space. If you assumed an average continuous spindle and accessory load of 10hp, or about 7.5kW, it would take a little more than 2 tons of cooling just to offset the heat produced by the machine.
All said and done, window units won't give you the best cooling system, but they will be your cheapest option by far. And, you can add them incrementally to the point where the shop is comfortable to suit yourself.