Duplicating Artboards

I only have 18 letter size artboards. Shouldn't I be able to have more than this?

You can create more artboards than that. But Illustrator's page treatment has always had the ill-conceived interface of positioning the initial page at the center of its limited pasteboard. This was not even changed when Illustrator very belatedly finally gained the ability to have more than one page in that limited space. So duplicated pages, by default, are created rightward and downward in that limited space. So you run into the right bounds of that space in half the time you should. That's why the message suggests you re-arrange the artboards.

By way of comparison, Illustrator's historic competitor, FreeHand, also consisted of a "page spreading" rather than a "page flipping" interface and had a similarly-sized limited pasteboard. But FreeHand's initial page was always more intelligently positioned at the lower left origin of that space, according to Cartesian 2D coordinate convention. So duplicated pages proceeded from the left, not from the center, before wrapping vertically, so did not prematurely run out of space.

When FreeHand gained its multi-page capability (long before Illustrator), its treatment was also much more useable. From the first introduction of the feature, multiple pages could be selected, aligned, distributed, and abided to grid snaps just like ordinary objects. There was no need to put the program in a page-manipulation "mode." There was simply a separate tool for selecting pages. It was entirely less tedious than Illustrator's belated treatment.

JET