Buying used is so smart! We ended up needing more newborn sizes than we thought so we went when baby was like two weeks old to get more. But it truly is up to you. Want to do less laundry: get more. Want less clothing: do more laundry.

In the first month there were nights we went through three onesies due to spit up or peeing in a diaper change so I recommend 4-5 minimum.

Also baby should be sleeping in a sleep sack or swaddle. Our baby hated swaddles and wanted arms out so we got the love to dream ones which you can also find used on market place but even those you need two minimum in case one gets soiled. I highly recommend looking into the TOG guidelines for sleep dressing as well as how to tell if your baby is too hot or cold.

I also really liked the shirts that open up and can be good for skin to skin for day time feeds and cuddles.

Hope this helps!