As the title implies, you can only be saved a total of 20 times or fewer during the whole Epilogue playthrough. Be warned that you will most likely NOT get this on your first run unless you know when and where to save and reload.
As with the similar achievement in the main game, dying means to have Elika reach her hand down and pull you out of dangers. These dangers include falling, touching wallenemies and staying too long in an area with polluted air.
Do keep in mind that if you're pushed down to the ground by an enemy in combat and fail to press the correct button in time also counts as a death.