Thirsties recommends these amounts based on washing every other day:

Newborn: 20-24 diapers
6-12 months: 14-18 diapers
12-24 months: 12-16 diapers
Potty Learning: 4-8 diapers

The number of diapers you need all depends on your baby's output. Younger infants tend to "go" frequently whereas children approaching the potty learning stage go through less diapers. As in everything parenting, you know your child best. Additionally, your cloth diapers will hold up better if you rotate their use over a decent-sized stash. The numbers listed here are a good place to start.

How many covers do I need?
A single Thirsties Duo Wrap or Thirsties Diaper Cover can be wiped clean and reused 3-5 times before requiring a full washing.

Newborn–9 months: 1 wrap for every 3 diapers
9 months–18 months: 1 wrap for every 4 diapers
18 months to potty: 1 wrap for 5 diapers

*Please note, not all diapers require a cover.  Many diapers, like Thirsties AIO's and Pocket Diapers include waterproof shells.  Diapers that require a separate cover include Flats, Prefolds, Inserts, and Fitteds.