21 · 8:28pm Mar 14th, 2015
21 is the number of canonical stories Maud Pie has appeared in (yes, Rainbow Rocks counts) times the number of attempts to bond with the Mane Six (not counting her sister, of course) that she had to go through to find one that at all worked.
Happy Pi Day, by the way. This was the best Pie-family related derivation of the number 21 that I could think of.