Hi! It totally depends on the brand. Usually the online store will indicate the age guide for each size or you can chat online or email the seller to know for sure what suits baby's size. However do note newborns will grow really fast. To be sure best to get clothes for newborns from physical stores. They will start from 0 mostly. You can also feel the texture of the clothes which is much more assuring then online purchases, at least that is what i prefer. Hope this helps.
best not too buy too many newborn size. in case your bb is born on the bigger side, den will skip newborn size totally. just buy a few will do, as they grow really quickly.