One would have to know the cfm of each carburetor to know how many you can add. They may not of had smaller units to choose from so 5 was the limit of what was available at the time. I would guess, out of the five, three may have been working and the other two came online at a higher rpm.
For example, my 250 ci had one single Monojet when new. I added a second one on a dual intake manifold. It would run, I drove it, seemed to have decent power, BUT it drank fuel faster then I could add it! I'm talking maybe 4 or 5 mpg.
I then switched to dual Carter W-1's off a 216 ci engine, a much smaller CFM rated. The smaller carbs kept the air velocity higher in each one so it still worked as it was designed to.
Carburetors are designed around a certain CFM rating, what the engine needed for everyday driving, and the occasional high speed run up. It doesn't matter what engine you put it on, the design and calibration does not change as long as the correct air velocity for the carburetor is met. Too much air speed or too little will change the metering, so if you can can keep the air speed right, you can pile on as many as you want.
