Do you mean the iOS YT app? It's at most three, two if you allow a swipe. Swipe down on the video (moves it to a smaller view in the lower-right) > tap the X in the upper right of the video. Or tap the video (to bring up controls) > tap the down-arrow in the upper left (moves to the same smaller view) > tap the same X in the corner.

Full screen:

1. touch screen to see controls

2. touch lower-right -><-

3. touch screen to see \/ in upper left

4. touch \/

5. touch X

If the video play to the end (1) and (2) are replaced with touch X, for a total of 4.