Displaying more Portfolio Items

Hello, I have 18 published portfolio items that I want to display on my homepage. In the theme customisation, under "Theme Options" I have been able to increase the default display from 5 to 6 posts and selected the 6 posts I want. However when I increase the number more than that (7 through to the 18 I need) no drop-downs show up for me to select the other items. In other words, I can see up until the drop downs for "Portfolio 6" but if I insert 7 in the "number of items to show" box, I don't get prompted for a "Portfolio 7" option. The purpose of the website is to display the portfolio, so I really need to be able to display all 18 items. Any thoughts?