As most new soon-to-be parents, I was researching and wondering about this question myself "how many diapers would my baby go through in his first year? How many boxes of nappies would I need to prepare or buy when its on sale??"
Early on, I already know that nappies (as I have chosen to use disposable nappies) would account for one of the biggest expense in having my baby.. so I really wanted to be prepared for it... and the number is... 2500!!! Yep, 2500 of nappies.. yep it is that much and no Im not exagerating ;)
Here is the rough guide and the break down for it though. Please keep in mind that of course every baby is different and grow at different rate too...
Newborn (birth - 6 weeks) : 10 per day, 70 per week, 420 nappies in total
Size 2 (7 weeks - 16 weeks) : 7-10 per day, 50 - 70 per week, 637 nappies in total
Size 3 (17 weeks - 30 weeks) : 5 - 7 per day, 35 - 50 per week, 595 nappies in total
Size 4 or Crawler (31 weeks - 1 year) : 5 per day, 35 per week, 805 nappies in total
After 1 year of age, the number of disposable nappies will decrease although still to a huge number of between 1500 - 1800 :)
Then also the next important things is baby wipes!! With each nappy change, you will only use 1 nappy per change (mostly.. unless your baby is anything like mine... they absolutely LOVES to do nature call just as soon as I've changed their nappies...) anyways, you will probably use more than 1 wipes.. and that means you will use roughly around 5000 wipes in your baby first year.
A tip for new parent : try to find a baby wipe that is a little bit thicker as it makes wiping really easy, cleaner and you wont ended up having to use as much as if you are using thin flimsy wipes :) Personally I really recommend Huggies wipes as that is what I've used with both my sons.
I hope this information is useful and you can plan on stocking up on the nappies when its on sale before the baby arrival :)