Newborn diapers go up to 10lbs but size 1 usually fits 8lbs. Big difference is that NB has the cutout for the umbilical stump, with size one diapers you can just fold them down in the front. I have 2 packages of NB and will not be buying more.
My son was only 6lbs 11oz, and we got a good amount of NB diapers from the hospital, but honestly we went almost straight to 1s!
My first was in newborns for at least a month. She was 6lbs 10oz at birth and struggled to gain weight for the first month
If im completely honest i didnt buy any newborn size at all the hospital gave me sooo many packs of the pampers maybe likw 4-5 by the time those were done we sized up to 1 i think size 1 is a safer choice if you want to buy a box
I got a ton of NB diapers from the hospital, so I only used one box of newborn diapers I find they stay in size 1 longer
I buy a big box of size 1 and then a few small bags of size 0 just incase
I just brought a couple of packages, it really depends on how fast you baby fills out and the weight when they are born :)