Furi

"Rising from a K.O." occurs when your health bar has reached 0 (your health bar is the larger series of rectangles in the top left of the screen), but you still have more than one of the smaller squares beneath your health bar filled. If you press a button while down like this, your health bar will glow green and refill to full, and one of the smaller squares beneath will become hollow.

That's 1.

Repeat that 20 times to unlock the achievement.

You will tend to unlock this naturally while playing the game. But if you would like to boost, you can do that by simply allowing any enemy to kill you repeatedly.

I don't know for sure if it counts as 1 when you lose the final smaller square and do not stand up again, but based on the description, I would expect not.

Rising from K.O.s in training mode (selectable from the main menu) does count.