The amount of cloth diapers you will need to buy depends on a few things; how frequently you change diapers (this is especially related to how old your baby is), how often you plan on washing, and what type of diapering system you choose.
When planning for diapers for a brand new baby, you will especially need to concentrate on how often you will be changing diapers. Because newborns tend to have a bowel movement nearly every time they urinate (especially those that are breastfed), they may need anywhere from 12-18 diapers a day!
For this age, I recommend either a prefold and cover system, fitted diapers with covers. For convenience with other diaper changers (My Husband) or when going out and about, you may choose to pick up a few pocket or all-in-one diapers.
So, when doing laundry every 1.5 days, for newborns I would recommend...
18-24 prefolds OR fitted diapers (or a combination of the two)
2 Snappi diaper fasteners (if using prefolds)
18-24 wipes (if you are washing diapers, why not wash your wipes too!)
2 pail liners (one can be washing and one can be clean)
For older Babies & Toddlers
Once your child gets past the newborn stage, you will need to concentrate much more on absorbancy, especially for toddlers. If you are doing laundry every day and a half, you will need 14-16 diapers, in any combination of prefolds, fitteds, pockets, or one-size diapers. As baby gets older, this stash could stretch you longer than a day and a half. Want to wash every two days? Add a few more diapers to your stash.
For the most economical solution, you can use prefolds and covers. Fitteds with covers or pockets add more convenience, and then all-in-ones are the most expensive but easiest to use. As with newborns, don't feel like you have to stick with one system! Any combination will be fine.
4-6 covers (for prefolds and fitted diapers)
18-24 wipes (If you are washing diapers, why not wash your wipes too!)
2 pail liners (one can be washing and one can be clean
As your child gets older, you will find yourself needing extra absorbency in your diapers. The time frame for this can vary depending on how often the diaper is changed and how much of a heavy wetter the child is. When this happens, you will need to add some extra absorbency to your diapers.
If you are using pocket diapers, you will need 1 insert (two for a heavy wetter) for each day time pocket diaper. For night time diapering, you will need 2-3 inserts per diaper.
For fitted and prefold diapers, you will need 1 cover for every 2-3 fitteds or prefolds. The cover can be reused as long as it isn't wet or soiled. For night time diapering, try a fleece or wool cover for maximum wetness protection.