After you've looked at your requirements and work habits, make a diagram of your office or home, showing computer locations, and indicate where you want to place special shared resources, such as modems, printers, a main machine, and the like. Remember that shared resources and the computers they're attached to need to be turned on to be used. So, for example, your office's main printer should probably be connected to a computer that will be on and accessible all the time.
You clearly need at least one workstation for each worker (or family member) who needs simultaneous access to a computer. Consider having an extra computer or two for the following: