Average human has capability to differentiate similar information into 7 groups. For example if people are asked to 100 glassless drinks sweet tea randomly with all of them is in different sugar level. Then they are asked to sort from the lowest level to highest level of sweetness. Averagely they will have 7 categories before making a mistake.
This is also the reason why the digit of phone number is limited to 7, because the average capability of human can store different number is 7. That is the magical of number 7.