"The magical number seven, plus or minus two" is one of the most cited works on psychology.
It tells us that a person can store in short-term memory from 5 to 9 (7 + -2) "chunks" of information.
Such a chunk may be a number or a word.
If there are more chunks, the brain tries to redistribute the information so that the number of chunks decreases to 9 or just "forgets" a piece or two.
For example, when we go to corporate party that organized by the company where our spouse is so successfully working, we can memorize up to 9 names of his/her collegues.
But if there are more of them we should remember only name of his/her boss, because we anyway will not be able to remember everybody.
Practical conclusion from the rule: The number of words in a sentence when training a speaking skill should be no more than 9, 5-7 is even better.