The number of designs for sheets refers to the entire sheet. You can have as many different pieces of artwork within this sheet and it still counts as one design! The number of designs refers to different sheets, so if you had one set of artwork on one sheet and then a different set on a second sheet, this would count as two designs and so on!
The maximum number of individual stickers you can have per sheet is 15. If you would like more than this on a sheet, we'd need to split them over a few sheets and charge these as extra designs.