There are a total of 50 Feathers in Assassin’s Creed 3. All of them are scattered across the Frontier. Most of them can be found on trees and only very few are cleverly hidden inside buildings or caves. You can buy in-game maps at the General Store that will reveal all Feather Locations. Buying the maps costs £10.000, so you might want to wait until you have enough money.
Collecting all feathers will unlock a new special outfit that can be equipped at the Homestead Manor. The Feathers are also needed to reach 100% overall game completion and are thereby a requirement for the Completionist trophy / achievement.