How many watt blulb do I need for a Black Headed Monitor?

While you can use UV lighting, it's not necessary. 45 watt halogen basking bulbs will do. Monitors grow quite large, so make sure the enclosure is roomy enough. It's important to provide a choice of temperatures so that the monitor can opt for heat or cool. In their native habitat, humidity is high, so mist the enclosure several times daily. Adding a floor of earth can help.