Outs are the cards that improve you to the best hand. A flush draw has 9 outs; an open-ended straight draw has 8.
Multiply outs by 2 for one card to come, by 4 on the flop for both cards, a quick estimate of your equity. With 9 outs the rule says 18% and 36%, the exact numbers are 19.1% and 35.0%.
After the flop, 47 cards are still unseen: 52 minus your two hole cards and the three on the board. No draw can have more outs than there are cards left, so 47 is the ceiling and it already means every remaining card wins.