You can use as many graphic cards and CPUs as you want with Iray if your motherboard support it, you just have to enable them in Iray hardware settings. However, if you want iClone to take advantage of more graphic cards I've been recommending that they add support for Vulkan or now, DX12 as they both offer multi GPU and CPU support + multi core CPU and GPU support bypassing the need for tessellation.

Also just a tip, most motherboard slots do not fully operate at 16x speed -after you add the first card, the other slots become 8x speed.