To me it depends on population density of the wall - will this be a unique piece on that wall, or will it be one of many? When hanging shows where I have limited linear wallspace, I'll double up and hang two rows of images. I try to keep at least 3-4" between frames horizontally as well as vertically so that pieces meant to be read individually can be, but that's not a hard and fast rule. It also depends on if the images are horizontal, vertical, or a mix of the two.