DMX is also shorthand for an intelligent lighting control protocol called DMX-512. The 512 means that there are 512 channels of control. Think of it as MIDI, except for lights. Your average fixture takes up between 8-16 channels, depending on the features. For example, a moving head fixture such as a MartinMAC 600 NT takes up one channel for intensity, one channel for strobe speed, one channel for pan, and so forth. So in other words, you run out of channels after maybe using 25 fixtures. That's why most controllers, such as the Wholehog series, and the Lightjockey, support multiple universes, allowing for more than 512 channels.