It depends on a few things. Mostly how big baby is when you deliver. My son was a preemie, when he finally grew into newborns he worn them for about two months.
Not many, newborns grow fast! Usually some outfits have the weight limit on them. I just have a few newborn/ 0-3 and the rest are 3-6 months, that's also what I'm asking for.
I have 15 and I think thats more then enough because they are so tiny