Maximum number of monitors Win 7 Supports?

There is no "limit" per se set by Windows. You are only limited by your ability to implement the hardware to support it. However, in order to make the maximum use of it and to make sure it works smoothly together, all of your video cards should use the same driver. If you start mixing different video cards/drivers it starts to have problems.

Windows does have a hard limit on maximum resolution and overall pixel count. The maximum continuous windows desktop available is up to 32k horizontal pixels by up to 32k vertical pixels with an overall pixel limit of 128 million pixels. Which means that a 32k x 32k desktop is not actually possible. Even video wall processors, which are generally purpose built windows dekstops, are constrained by these limits.

Max pixels >= horizontal pixels(H) x vertical pixels(V)

128 Million >= H x V

If you want to setup nine monitors, you'll certainly have to look beyond consumer products.

With Cinemassive Video Wall you can drive up to 64 monitors on a Windows machine.