Use 15 gallons of water total in this recipe.
Based on boiling 13.14 gallonsliters at 1.050 for 60 minutes, this will decrease your wort by 1.50 gallonsliters, bringing your after boil OG to 1.056. You were planning on an OG of 1.056 so you are right on track. Nice job!
Use 15 gallons for this recipe.