Many moms recommend having a minimum of 15 onesies with a mix of both long and short-sleeves. In addition, it would be good to have at least 7 sleepers (full-body pajamas with the footies) for nighttime wear.  This amount of onesies allows you 2 outfits per day and one set of baby pajamas per night for a whole week.  With any luck, you might only have to do laundry for your newborn once a week!