OS X Server has always been something of a bargain compared to the various flavors of Windows Server. Unlike Microsoft, Apple never focused on a client access licensing model in which organizations must pay for the server software itself plus additional licenses for users or devices that connect to it. Apple also doesn’t break OS X Server down into multiple variations each with its own features, licensing needs, and upgrade limitations.
When you buy OS X Server, Apple gives you everything from file sharing to Internet and collaborative services like wikis and internal messaging through Mac and iOS device management. If you start as a small business with a single basic server and eventually grow to the point where you need to support and manage dozens or hundreds of Macs, PCs, and mobile devices, there are no limits imposed on licensing or data migration..