This can be done if the category supports listing variations (not all categories do). If it does, underneath the title field should be a link that says "create variations for this item". If you click that, you'll be taken to a screen where you can create and name an attribute. Because there is a quantity field in the listing regardless, eBay won't let you create an attribute labeled "Quantity", so you have to name it something else (popular variations are QTY, Lot Size, Pack Size and so on).
Once you've created attributes, you can then create and add variations (in this case "1pc" and "5pcs", or whatever the best name for those). Once that's done, you can continue on to the next screen where you upload photos, set quantities per variation, upload photos etc.
If the category you need to list in does not support variation listings, you need to create two separate listings - one for each variation.
