Microsoft AD 

In Microsoft Active Directory (AD), a "directory" refers to the hierarchical structure that the service uses to organize networked resources, such as users, computers, groups, and other resources. The directory serves as a central location where all this information is stored, making it easier for administrators to manage network resources and for users to find and access these resources.

Here are some key components of the Active Directory:

1. Domain: A domain is a logical grouping of network objects (computers, users, devices) that share the same Active Directory database.

2. Tree: A tree is a collection of one or more domains and domain trees in a contiguous namespace, linked in a transitive trust hierarchy.

3. Forest: A forest is a collection of trees that share a common global catalog, directory schema, logical structure, and directory configuration. The forest represents the security boundary within which users, computers, groups, and other objects are accessible.

4. Organizational Units (OUs): OUs are containers within a domain that can contain users, groups, computers, and other OUs. Administrators often use OUs to group related resources together for easier management and application of Group Policy.

5. Objects: An object is a distinct, named set of attributes that represents a network resource. Objects can be users, computers, printers, applications, or any other resource that can be part of a domain.

6. Schema: The schema is a set of definitions for all the objects and attributes that can be stored in the directory. It also defines the rules for how those objects and attributes can be created, modified, or deleted.

7. Global Catalog: This is a distributed data repository that contains a searchable, partial representation of every object in every domain in a forest.

These components of Active Directory allow for scalable, secure management of network resources in a variety of different network configurations

The number of Active Directory (AD) domains and forests (referred to as 'directories') an organization should have depends on various factors. While it is generally best to keep the number of directories as small as possible to simplify administration, the following factors might necessitate additional directories:

1. Security boundaries: Each AD forest represents a security boundary. If a business unit requires total autonomy and separation of security information, a separate forest might be necessary.

2. Organizational structure: In a large, decentralized organization, it might make sense to have separate AD domains or forests for different departments, subsidiaries, or geographical locations.

3. Mergers and acquisitions: When companies merge or when one company acquires another, it's often necessary to create additional directories to integrate the IT systems of the separate companies.

4. Regulatory compliance: Certain industry regulations or legal requirements might require that data for certain business units be kept separate from other data, necessitating separate directories.

5. Performance and Replication: Network traffic, database size, and replication latency could be factors that necessitate additional domains. These are usually concerns for large, multinational corporations with complex network infrastructures.

6. Disaster Recovery: Having multiple domains might be beneficial for disaster recovery purposes. If one domain fails, other parts of the organization can continue functioning.

7. Application Requirements: Certain applications may require their own Active Directory forests or domains to function correctly.

However, it's important to note that more domains and forests mean increased complexity in terms of administration, cost, and potential for security issues. Therefore, a balance needs to be found between the need for separation and the need for simplicity and efficiency. Today's best practices suggest minimizing the number of domains and forests whenever possible.

Remember that Organizational Units (OUs) within a single domain can also provide a way to delegate administrative rights, apply Group Policies, and organize resources, without the additional complexity of multiple domains or forests.

A general best practice, however, is to minimize the number of forests and domains. A single AD domain can support millions of objects, and with good design, it can meet the needs of most medium-sized organizations.

A single forest, single domain model simplifies administration, reduces replication traffic, and simplifies the design of the directory's logical structure. If further separation of administrative duties or application of different Group Policies is needed, this can often be achieved with organizational units (OUs) within a single domain.

It's also worth noting that the advent of cloud directory services like Azure Active Directory and AWS Managed Microsoft AD has changed how organizations structure their directories. These services can provide scalability and flexibility, reducing the need for multiple domains or forests.

However, every organization is unique. Factors like regulatory requirements, business structure, geographical distribution, security requirements, and the specific applications used can all influence the design of an organization's AD infrastructure.