Build a Website

I have a shopify website. There are a few collections (categories) that I would like there to have the quantity to be set in multiples of 2. For Example, all items in category x, must be purchased by in 2s,4s,6s... etc. Basically they are for earrings and I would like to set the price to be for 1 but they have to buy in 2s. These same items will also be in the "new arrivals" category but with other items that don't need to be purchased in 2s. But if the customer were to click on the earrings in the "new arrivals" it should still show quantity in 2s.