29.97 is the number. If you count frames in drop-frame, the amount of time it takes to play the clip is more accurate due to not counting certain timeslots. And when you have an exact amount of time between television commercials, this can be critical.
If you count 30 frames every second, eventually the timeline and the actual time eleapsed will be far enough off to cause a problem.
Either way …. the frames are all played. It is the accuracy of the elapsed time that is the issue.