I'd avoid buying too many in one size/brand. My DS wasn't in first size for long at all but spent a long time in second size but grew out of them pretty much in the blink of an eye. We also found Pampers really leaky and just didn't work for us. If your dog it by offer look at price per nappy rather than overall price, a few times I've noticed they're not actually the best deal on the aisle.
As you know it all depends on the size of baby and how fast they grow.
Newborn size will last for a few weeks at least even on a big baby and estimate 4-5 nappies a day minimum, some days you'll need more some less again all depends on baby. If your planning on breastfeeding new baby's tend to poo after each feed.
Even if you get the bigger sizes and store them they will be used at some point. Think about nighttime nappies for the bigger sizes as they will help keep baby dry and hopefully you'll get the best sleep possible.
You may develop a preference for type/make as time goes on.
I agree with pp don't stockpile, if one of the big supermarkets doesn't have nappies on offer the others do. I also agree that newborns get through 10-12 nappies a day.
My first started in size 2 nappies and moved to size 3 at about 6 weeks, but my babies are big, I am due in 6 weeks and currently have 2 boxes of size 2 nappies, will buy another box/2 when I finish the first.
Also it was good last time to try out shops own brand nappies/various big brands before I decided on what suited me/my son for instance Huggies were a no no as poo just came straight out the sides of them, but i knew other mums swore by them.
Different brands of nappy seem to suit different shaped babies. Pampers were good for us newborn size but Huggies leaked. I've heard other people saying exactly the opposite though - ds had really skinny legs which I think affected the fit.
I've bought four packs of size 1 in different brands/supermarket's own depending what I saw cheap and will stop at that. Once you find one that suits you it's cheaper to buy the big box/huge pack anyway.
I agree with the others, no need to stockpile, the prices don't change much and all the brands are regularly on offer in one supermarket or the other. I had an average sized 8lb baby. He spent about 10 days in size 1 and then went into size 2. TBH he could have started in size 2 but since I had the 1s and they had the handy cord stump cut out bit, Iused up the packets I had bought. I think I had about 3 packs of size 1.
I found different brands better at different ages. Tesco and Boots were good when tiny, Pampers and Huggies leaked sometimes, have found Lidl best and price is consistently low even compared to main brands on offer. We also buy Lidl baby wipes at 78p a packet. Main brands tend to make DD's bottom sore - cotton wool and water is best for new born though.