Argument introduced by Miller, G. A., suggests that the number of objects an average human can hold in short-term memory is 7 ± 2.
From a practical standpoint, this means that information designed for quick perception must contain no more than 9 elements @ each level, when planning strategy, workflow, process map, container strategy etc.
 Miller, G. A. (1956). “The magical number seven, plus or minus two: Some limits on our capacity for processing information”. Psychological Review. 63 (2): 81–97. CiteSeerX10.1.1.308.8071. doi:10.1037/h0043158. PMID13310704.