It is recommended to have at least two domain controllers for fault tolerance with every active directory deployment. Just having one domain controller is not a good idea. DC’s don’t need to be very beefy, so you can get by with basic server configurations. For 1000 users, I would recommend 2 to 4 domain controllers depending on what applications the users run. For 6 locations, I would recommend that you go with at least 3 and have one located in one of your larger branch locations.