How many cloth diapers you need depends on the age of your baby and how often you plan to wash diapers. You will need fewer in each size as you baby grows. The type of diapers you choose will also influence the number of diapers you need.
Some styles will need changing a bit less often. For instance, pocket diapers have a moisture resistant barrier between the baby and the absorbent insert. This means you can change baby less often unless the diaper is dirty so you will likely need fewer than for a less absorbent option.
If you have a heavy wetter your baby will be on the high end of the number shown. You can extend the absorbency by using a doubler, using two prefolds or using newborn prefolds as doublers for older babies. With my own daughter, I often use a hemp and micrifiber pocket insert combination so she stays dry all night and during long naps. Since I can't always predict her naps, I use this combo a lot during the day and it works great for keeping her dry.
While one size diapers are popular right now, many parents prefer the less bulky fit of sized diapers. Since your baby is likely to only need two sizes and will need only a few of the larges if they happen to get that big, sized diapers offer a great option for parents.
Cloth diapering will save you a lot of money, but the up front investment can be intimidating for parents who are unsure if cloth is the best choice for their families. If you can't afford to invest in all you need all at once, you can start with one day's worth and then add more the next time you get paid. Just take the amount you are spending now on disposables and save it to purchase more cloth once a month until you have the number you need.
You will want to get a few extras to have on hand while you are washing diapers. Nothing is worse than having a sick baby and no clean diapers.
Up to 6 Mos. 10-12 per day
6-12 Mos. 8-10 per day
12-24 Mos. 6-8 per day
Potty Learning 2-4 per day
You will need one diaper cover for about every 4-6 diapers you have. Most covers can be reused a few times by airing them out between changes unless they are dirty. It is always a good idea to have one more cover than you think you need in case one gets dirty at a bad time!
Wool covers can go even longer between washes if they are properly lanolized when they are washed. PUL diaper covers are easily cleaned and dry quickly if the need to handwash one arises between loads of laundry.
Pockets and all-in-ones do not need covers. You will need to have at least 1 insert for each day time pocket style and 2-3 inserts for each night time diaper. Hemp is a great choice for extra absorbency for heavy wetters or overnight.