Multiple monitors let you extend your Windows desktop across more than 1 monitor for extra screen real estate. Each additional monitor requires its own video card. The exception being if you have special multi-monitor video cards .
Additional adapters require specific driver support.
Anywhere from 2 - 9 monitors are supported. Each monitor can be set at its own colour depth and screen resolution . Windows lets you specify the position of each monitor with respect to one another. For example, Microsoft Flight Simulator has multi-monitor support, but secondary display views must be in windowed mode. Hopefully future versions of DirectX will solve this dilemma.