The number of users as a requirement for your Lion Server machine
The impact of an increasing number of connected clients on OS X Lion Server performance is not linear. You may not notice a slower service as more clients connect until you reach a tipping point, when performance suddenly slows to a crawl.
Mac mini can handle a maximum of 20 to 50 client computers for lightweight tasks. The more services that consume a lot of devices, the lower this number.
With up to ten or so users, minimalist Macs can handle multiple tasks simultaneously. If you add more users later, you can always add more down-level Mac servers for other tasks. Current iMac models have faster processors and architecture than smaller models, can accommodate more RAM, and can handle more clients.
This best-in-class Mac Pro can handle hundreds of clients, again depending on which services you’re running. With an older Mac Pro or Xserve, adding more high-end storage or large amounts of RAM can help increase the client load that the server can handle. Network capacity (number and speed of Ethernet cards) is also important in serving large numbers of customers.