Optiplex 990 SFF Graphics Card Upgrade?

Hi I have an optiplex 990 SFF with Windows 10 and am looking to hook it up to four monitors (eventually 6). I bought a new graphics card for it, but it was too big and I had to return it. I can't seem to find anything that looks like it will fit, and also as 4-6 monitor support. Any help would be greatly appreciated, thanks.