10 Computers & 10 Users

From time to time, people ask the home server team how and why we made certain decisions for the initial release of WIndows Home Server. Currently, you can define 10 user accounts in the Windows Home Server Console and you can install the Windows Home Server Connector software on up to 10 home computers running Windows XP or Windows Vista.
In all of the secondary research that we reviewed and primary research that we did for home server as part of the product planning process, it was very rare to find broadband connected households and home-based businesses with more than 10 people and with more than 10 home computers. Additionally, Microsoft offers a great product, Windows Small Business Server, that scales well beyond 10 users for more sophisticated home-based businesses or small businesses that plan on growing. You can read about the upcoming release of Windows Small Business Server 2008 on the microsoft web site.
We didn't want to build a consumer product that used CALs (Client Access Licenses) as we really didn't think consumers wanted to deal with managing licenses for their home PCs and sometimes when you say CAL, people hear "cow" and respond that they live in the city not on a ranch and don't really have a need for cattle.
However, we knew that there would be rare cases where someone had 11 computers or 12 or 17 or ? in their home. So, long ago we made the decision that a user could have 2 home servers, where a given home computer would only be "joined" or "connected" to one for the purpose of the daily automatic image-based backups and centralized health reporting through the Windows Home Server Console.
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Reference : http://blogs.technet.com/homeserver/archive/2008/09/04/10-computers-10-users.aspx