It would then seem to suggest, that forces with more than say, 5 units (780 average sustained APM) and above, would likely be getting into super human territory.
You can also hotkey individuals or groups, 1-8. Again just rolling through those will get you a fair amount of apm.
That said, 780 is peak for world class players. You probably can't do that, ever, because by the time you put in the years of effort, your reflexes will have decayed.
On the other hand, brood war was a subtle game. APM is not the be all end all of winning. Getting proficient with a few aspects of the game can make you pretty good pretty quickly. Heck, just looking around the map like i mentioned above will help. Some people are amazing controlling a few units like a surgeon. Those units are practically unstoppable. But they miss the big picture. Scanning the map lets you distract, delay and ultimately minimize the value of those unstoppable units.