Its always been a questiom of mine ever since I got the game. Does the number of boxes increase from 8 each time you get more Pokemon or does it not? The only way I have found of doing this is by doing the Pokemon Bank Glitch.
You will get more boxes if you have at least one Pokémon in each box. The boxes will increase from 15 to 23 to 30 and eventually to 31 after you capture Xerneas/Yveltal. You cannot however get more than 31 boxes.