Warning: Last items in stock!
Putting your portfolio together is a balancing act between making sure you get enough pieces in but not overloading with average content. Remember quality and not quantity is the key.
In most cases lecturers will have formed an opinion within the first 6 pieces you show and may not even bother to view the whole portfolio. If you feel some work is weaker than others then space these pieces out. You need to make sure you have uniformity in your portfolio i.e all the mounts are the same size, borders are the same width and that you can talk confidently about each piece.