Keeping the Same Number as Dummy

The exhortation to save the same number of cards in a suit as dummy shows is, like all the other maxims of bridge, not adduced as an inviolate rule to be followed blindly. For starters, if your highest card is lower than dummy's lowest, well, you can't capture anything there. Also, if declarer has no entry to dummy, it cannot be a threat and indeed, that's the very suit you want to get rid of (except top winners), though to be sure, evaluating an entry to dummy can't always be as certain as noting that your highest can't beat dummy's lowest.
Nevertheless, with qualifications aside, a failure to follow this principle, is a very common cause of giving up a trick, which at times is going to be a very valuable trick, giving up the potential to defeat a contract. Below are some links to hands where East didn't choose to keep the same number to his considerable disadvantage.
And incidentally, the same principle holds when you are discarding after the closed hand on the run of a long suit from dummy. Keep the same number as the closed hand holds -- except that, of course, you can't see the closed hand's holding. Nevertheless, you can watch the discards and often make an educated guess about declarer's original holding and discard accordingly. Maybe you'll guess wrong, but then again, you might be right, and it beats taking no account of what declarer is discarding.

One Two Three Four
Five Six Seven Eight
Nine Ten Eleven Twelve
Thirteen Fourteen Fifteen Sixteen
Seventeen Eighteen Nineteen Twenty
Twenty-one Twenty-two Twenty-three Twenty-four
Twenty-five Twenty-six Twenty-seven Twenty-eight
Twenty-nine Thirty Thirty-one Thirty-two
Thirty-three Thirty-four Thirty-five Thirty-six