Different presses and press/folder configurations can have different numbering schemes. Probably the most common, at least in my experience, is 1,4,5,8,9,12,13,16...etc on the front and 2,3,6,7,10,11,14,15...etc on the back but there are many more possibilities. It isn't hard to do something in spreadsheet software like Excel or the spreadsheet program in OpenOffice.