Determining outs:

The number of outs you have is the number of cards that will give you the best hand, for example, when you have an open ended straight draw, 8 cards out of the 47 that you haven’t seen will give you the straight, but not necessarily the best hand.

When calculating outs, it's also important not to count the same outs twice. An example would be a flush draw in addition to an open ended straight draw. 9 cards will give you a flush, and 8 cards would give you a straight, but since 2 of the cards that would give you a straight would also give you a flush, you have 15 outs, not 17.

Also, sometimes an out for you really isn't a true out. An example would be chasing an open ended straight draw when two of another suit are on the table. In this regard, where you would normally have 8 total outs to hit your straight, 2 of those outs will result in three to a suit on the table. This makes a possible flush for your opponents. As a result, you really only have 6 outs for a nut straight draw. Another more complex example is when you are drawing to a straight that is not the nuts. If you hold 76s and the flop is 789, a 6 or a 10 would give you a straight, but the 10 also would make a straight for someone holding a J, giving you 4 outs, or you could be drawing dead against a player who is holding J,10.

This is why I recommend only drawing when you are drawing to the nuts.