You can handle four variables at a time

How much can your mind keep track of?

Another limit to human cognition:

Halford et al concluded from these results that people -- academics accustomed to interpreting the type of data used in the experiment problems -- cannot process more than four variables at a time. Recognizing these human limitations can make a difference when designing high-stress work environments--such as air-traffic control centers--where employees must keep in mind several variables all at once.

This seems to be a "hard limit", if true then I'd guess that the cognitive burden grows exponentially with the number of items tracked. In that case five items might be vastly harder to track than four.

It's interesting to think how this might limit our ability to manage certain types of problems.