18 · 5:39am Mar 18th, 2015
18 is the number of stars that shall (did) aid in her escape, assuming you take an agnostic view as to whether the stars in question are in the sky or on Twilight's butt.
I would use the number of times she will bring about night at some point, but she's going to be doing that eternally, so it's a bit beyond the scope of this countdown.