I heard there was a limit GPU OpenCL can support depend on OS.
OS can detect as many as I plug in. But on Windows, I'm sure 100% I can only mine with 4 GPU.
But with Linux, someone said 6, someone said 10 (Ubuntu).
In Ubuntu 11.04, i mined with 8 cards on MSI big bang marshal mother board. 4 * 5870 & 4 * 6970.
So, i am sure In Linux you can mine with 8 cards that too different series.
Many says 11.6 catalyst allows Windows to mine with 8 cards, but it didn't work for me. May be if you use same series & cards, it may mine.