Sounds like you might be setting up a trading system.
If so, you do not need much graphics power from expensive gaming cards.
Crossfire is intended to apply the power of two graphics cards to a single gaming app.
One easy way to get a large number of displays would be to use multiple inexpensive graphics cards.
They can be about $30 each. They can use pci-e x16 slots, which are the cheapest, but pci or pcie-x4 too.
Each will drive at least two displays, but perhaps 3 if you have displayport monitors and adapters.
If your motherboard had integrated graphics, you can use their two adapters also.
If I assume that 7 screens are 1080P each, then you want to display about 14m pixels.
That will take up a lot of real estate on your desltop, or more likely your wall space.
A better, but more expensive approach might be to use 4 2560 x 1600 27" monitors, giving you 16m pixels.
The quality of those $1000 monitors is superior, and you will have better viewing angles.
In between, there are 2560 x 1440 monitors that cost about $700 each. 4 of them will total the same 14m pixels that the 7 monitors would have.
I would be inclined to go with this approach if budget permits.
And... you would only need two inexpensive graphics cards.