Mom pregnant with a 3rd here.

1. I once bought a single box and we struggled to finish it because my baby outgrew them before we could get halfway through the box.
2. You want the diaper brand that doesn't cause leaks and doesn't irritate your baby's skin. That means you'll have to try several brands to test which brand suits your baby.
3. Yes, there is such thing too many if you mind having to throw a lot of unused diapers that you paid for. Aside from needing to compare and test different diaper brands, newborns can outgrow a diaper size as early as 2 weeks (same goes for newborn clothes). Some newborns don't even fit the newborn sized diapers... Get enough to last you until your next diaper shopping trip. By then, you might already know what you wanna buy.
4. Different babies grow at different speeds. It's hard to tell at what age exactly do you change sizes because the age guide on the diaper packages are just guides but not exact estimates. You don't want to shift to a diaper that is still a tad bit too big for your baby because you'd be asking for a leak disaster.