The movies and animations that we watch are generally shot 24 frames per second, i.e. 24 fps.
If we draw 24 drawings for one second, with each drawing held for 1 frame, the animation is said to be on Ones.
If we draw 12 drawings for one second, with each drawing held for 2 frames, the animation is said to be on Twos.
While animating, you need a combination of both. It is best to animate on twos while reserving ones for very fast actions.
So while numbering your drawings on the x-sheet while animating on twos, always use odd numbers, subsequently skipping every next row; and when you want to smooth something out or need a very fast action, just add in the ones with even numbers.
If you want to hold a drawing for a specific number of frames, just indicate it with a straight line, and then get back to the next odd number respective to the frame number.