You should make it loop so the next last brick is the first brick.
Also I noticed it's also add-on based actually, so if you have 2 add-ons in one subcategory, it will only take the add-on pack the brick came from. So if you have 2 add-ons that use one subcategory, you need to have 2 bricks for the 2 sets in the subcategory. That's actually quite useful on some parts.