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

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 include a waterproof shell.  Diapers that require a separate cover include Flats, Prefolds, Contoured, Inserts, and Fitteds.