Having multiple DC's is a best practice standard. You should always have at least 2 DC's to safe guard against one going down. If all you had was one, and it went down, there'd be nothing for your objects to authenticate to.
It's very common for environments to have more than 2 DC's as well. There are a lot of factors which go into deciding if a new DC is needed. For example, you may have more than one physical location and want to have DC's on each site. When DC's enroll into an existing domain, they're able to find each other and share information.
You can also have multiple forests (groups of domains) linked together. You can do this by attaching them to the same "site" (using a directory manager like IDM) or by establishing trusts between the separately configured domains. However, most needs are satisfied by simply creating a new OU in an existing forest. You'll usually see multiple domains configured together as part of mergers and migrations or because someone had an older Novell IDM infrastructure.
Given the freedom which Virtual Machines offer, I'd highly recommend you get something like virtualbox and download a release preview of Microsoft Server 2012 R2 (currently free) and build your own domain. Feel free to play around and readh through Microsofts online tech databases.