A 7 pound baby can usually wear a size 1 diaper.
Most babies need a newborn size diaper until they reach 8 lbs and then the babies usually move up to a size 1 diaper.
But all babies are different and your newborn may need a size 1 diaper before other newborn babies do.
You usually only need 1 or 2 packs of newborn size diapers because the babies do not stay in size newborn diapers too long.
A baby will then need a size 1 diaper for at least 3 to 6 months and then they move up to a size 2 diaper and then a size 3 diaper and then a size 4 diaper and so on and so forth.
If the diaper fits too tight then they need a bigger size diaper.
You should be able to fit a few fingers into the waistband of the diaper and if not the diaper is too small and you need to get a bigger size diaper.