If I buy multiple Xbox 360 Wireless Gaming Receivers will I be able to use them at the same time? I know that each one should support 4 controllers, so if I buy 2 could I connect 8?
You can connect them, and all the controllers will show up as devices on your PC, but you won't actually be able to use more than 4 controllers.
XInput (the API that Windows games use to interface with Xbox 360 controllers) only supports a maximum of 4 controllers. Even if there were a way around this, games are designed with this limitation in mind.