[user5994461]

People can remember only 4 things. The few who can remember more are the exception.

That would be a much better goal for product design. You don't want designers to pack things by 7 or 9. It's too many.

[isostatic]

I tend to remember 3 things when it comes to things like shopping or tasks. Add a fourth and I usually end up getting something wrong or forgetting it.

[walshemj]

I think the average is 5, one of the diagnostics for Dyslexia is not being able to remember less than this in short term memory