For symmetry, that's 16 bookshelves including back corners. If you don't want to make an extraneous bookshelf, you could leave a block out of a back corner (which in many cases would be hidden against a wall anyway).
Or if you wanted symmetry without exceeding the cap of 15, you could keep both back corners intact and just replace one of the center bookshelves with something cool-looking (chiseled sandstone?).
If you currently have a 15/16-bookshelf enchanting room, what's your layout?