How many servers do I need to run a CollabNet TeamForge site?

You can run TeamForge on one server or split up its services among multiple servers.

TeamForge functionality is delivered by several discrete services. Each service can run on its own machine or share a machine with one or more other services. You assign specific services to specific boxes when you customize your TeamForge installation by editing the site-options.conf file.

TeamForge core functionality: This is known internally as the app server. It implements JBoss and Apache services. One and only one instance of this application must be present.
Database: The database application handles site users' data. You set the type of database by setting the value of the DATABASE_TYPE token to oracle or pgsql. One and only one instance of this application must be present.
Subversion: Subversion can be used to provide source control functionality. It uses the Tomcat and Apache services. A site can have zero, one, or more than one instances of the svn application, running on an arbitrary number of machines.
CVS: CVS can be used to provide source control functionality. It uses the Tomcat service. A site can have zero, one, or more than one instances of the cvs application, running on an arbitrary number of machines.
etl: The reporting service.
datamart: A mirror of your site database for the reporting engine to work with.
Orchestrate: TeamForge Orchestrate is a TeamForge capability that provides traceability for product life cycle activities such as work items, SCM commits, continuous integration (CI) builds, and code reviews. See Orchestrate's Installation requirements for more information.
Git: TeamForge 8.2 supports an integration with the Git distributed version control tool powered by Gerrit. TeamForge 8.2 supports Git integration based on Gerrit 2.11.x (version 8.5.x of the integration). For more information, see Set up Git.
Black Duck Code Sight (BDCS): TeamForge 8.2 supports the Black Duck Code Sight source code search engine. For more information, see Set up Black Duck Code Sight.

In principle, services can be combined in any configuration, with some constraints, such as:

Only one site database, datamart and reporting service.
Multiple source control integration services, but only one per box.

In practice, CollabNet has identified few configurations as the most useful for a wide variety of site. You can follow the instructions here to set up your site in one of these configurations, or you can adapt one of them to your own conditions.