Maybe 4, but it's hard to say. The motherboard might disable pci-e slots when adjacent slots/features are active. Without more research, or even hands on testing, there's no way to say for sure.
That is a horrible choice for a GPU miner motherboard. I can't comment on CPU mining.
dell precision 690 motherboard
Yeah, that is an additional problem. You'll need to search dell for manuals on that motherboard, but being an OEM manufacturer I doubt they make that information available. You will have to test and see, that is about it.
Again, a horrible choice for a mining motherboard.