A newborn can go through several outfit changes a day, from spit-up and leaky diapers alone. Six to eight onesies is a reasonable number to start with if you plan to wash every two to three days. Fewer than that and you are doing emergency laundry at midnight. Any more and most sit folded in a drawer before your baby outgrows them, since sizes move quickly from newborn through to 4 years. Epic sources double-layer muslin cotton onesies, pre-washed before they ship, so they already feel soft with flat seams against the skin from the first wear.
Washing before the first wear is worth doing even on new clothes, and whether new baby clothes need washing before they're worn covers why. Once you are in a normal rotation, wash cold on a gentle cycle and tumble dry low or hang to dry. Skip fabric softener and high heat. Muslin cotton gets softer with every wash instead of wearing thin, and how many washes it takes for muslin to feel its softest walks through what to expect in the first few weeks.
Onesies carry most of the early weeks because they are quick to change and keep a wriggling newborn covered. As your baby grows into rolling and sitting, two-piece outfits often take over for part of the day, and when babies move from onesies to two-piece outfits is a useful guide to the timing. Until then, a small stack in true newborn and the next size up covers the stretch where growth happens fastest.