Your AMD motherboard MUST be SLI capable I run an AMD FX 8650 OctaCore
The motherboard box will say Nvidia SLI compatible, if it's a Dell, HP or prebuilt I'm pretty sure it won't be SLI compatible
You can run as many as your power supply will drive and motherboard will hold but I found, for me, Dual cards work better, Crysis on ULTRA 1080p is a very playable 55FPS world of Warcraft Leigon on ultra is 200+ fps sustained 78FPS on Fallout 4
Good luck