Board Thread:M.U.G.E.N Help/@comment-26251721-20150511004115/@comment-3495717-20150511111350

The way screenpacks work is that they have a set number of character select slots based on two numbers defined in system.def for rows and columns. To increase the number of slots, you'd need to increase the numbers, though that does mean the appearance of the select screen may look odd, so you'd need to shrink the boxes, realign them, etc., which most people cannot be bothered to do, which is why most screenpacks nowadays have plenty of slots so you don't run into this problem; even the default M.U.G.E.N screenpack has a bigger variant (aptly named 'big') that gives you 60 slots.