When in doubt about a number, make it less than 7, +/- 2. This is the number of things that a person may juggle in their head at once. It is given as a range because it varies with the individual, circumstances, and particular task at hand.

The original source of this phrase seems to be this paper from 1956, by George A. Miller. An interesting point of the paper is that there are really multiple sevens: both our ability to distinguish information (bits in a chunk) and number of chunks that can be stored at once in short term memory. I had pretty much been lumping to the two together up until now.

More recent theories suggest that the \x91real' number is closer to four, with some mental tricks going on to increase our performance.

The theory of working memory has a completely different mental structure. Under this model, seven or so just happens to be the number of things the mind can say to itself in it's short term memory loop.

While all of this speculation on the real mental processes is fascinating, 7 +/- 2 makes a good rule of thumb for games, in much the same way as one drives a car without understanding the gory details of the internal combustion engine.