This depends on the forest recovery approach you choose for your environment. For more information, see Forest recovery approaches.
The decision on how many domain controllers to back up (and, therefore, which method to use for forest recovery) depends on the factors like
The overall number of domain controllers in Active Directory
The size of Active Directory database
With a large number (50+) of domain controllers in the domain and significant size (over 1 GB) of the Active Directory database, it may not be feasible to fully back up Active Directory of all domain controllers in the domain due to storage limitations or time constraints. In this case, you can back up only some of the domain controllers.
A good practice is to back up at least two domain controllers in each domain in the forest. It is recommended to back up the domain controllers that are DNS servers and FSMO role holders.
The Recovery Manager Console allows you to group the computers you want to back up into computer collections, each collection having its own backup creation parameters and schedule.
All computers in a computer collection are backed up simultaneously. The backup creation process may be a resource-consuming task if the number of computers in a collection is more than 10. Therefore, it is recommended that you back up only one computer collection at a time. Also, it is not recommended to have more than 10 domain controllers in a single computer collection.
All of the domain controller backups that you plan to use for forest recovery include the following Active Directory components:
Registry, including all registry hives and the NTUSER.DAT file
On the Advanced tab in the Properties dialog box for the computer collection, the When backing up Global Catalog servers, collect group membership information from all domains within the Active Directory forest option is selected by default. However, if the backup creation operation takes a significant time to complete, you may disable collecting group membership information from all domains within the forest.
To streamline the creation of Active Directory backups, you can follow these best practices:
Avoid using groups with cross-domain membership in Active Directory as much as possible. To reveal such groups, you can use an Active Directory reporting tool such as Quest Reporter.
If you cannot avoid using groups with cross-domain membership, make sure you have a procedure in place to back up and restore these groups manually. For example, you can do so by using such command line tools as Ldifde or Csvde provided by Microsoft.
For each computer collection you can specify the data compression method to be used in backup creation operations. To ensure both a reasonable backup time and a reasonable compressed backup size, it is recommended that you use either Fast or Normal compression method. For more information on how to specify a data compression method for a computer collection, see the User Guide supplied with this release of Recovery Manager for Active Directory.