How many GPUs does the GA-X79-UP4 support?

The GA-X79-UP4 sports 4 PCIe 3.0 x16 slots and appears to be able to handle double width cards for all slots. The first two slots run at the full 16x, the last two at 8x. The specs are not GPU specific.

AMD Radeon 7950 specs:

850MHz Engine Clock (Up to 925MHz with Boost)
3GB GDDR5 Memory
1250MHz Memory Clock (5.0 Gbps GDDR5)
240GB/s memory bandwidth
2.87 TFLOPS Single Precision compute power
717 GFLOPs Double Precision compute power
GCN Architecture
28 Compute Units (1792 Stream Processors)
112 Texture Units
128 Z/Stencil ROP Units
32 Color ROP Units
Dual geometry units
Dual Asynchronous Compute Engines (ACE)

PCI Express 3.0 x16 bus interface

DirectX® 11.1-capable graphics
9th generation programmable hardware tessellation units
Shader Model 5.0
DirectCompute 11
Accelerated multi-threading
HDR texture compression
Order-independent transparency
OpenGL 4.2 support
Partially Resident Textures (PRT)
Ultra-high resolution texture streaming technology.

Alright the specs for my GPU. So you said it can sport 4 x16 slots and the 7950 appears to be x16 (I singled it out in the specs list). So I can take 4 of them at once? You later said only two of the slots are actually x16 and two of them are x8 which left me a bit confused.

When running multiple PCIe x16 slots usually the first slot is the only one capable of actually running at x16 throughput lanes. The socket 2011 with X79 allows for two slots to run at x16 (that's good) or any single slot at x16 (that's unusual) but once you get past the first two slots occupied the system can no longer provide the bandwidth to the PCIe slots hence they do not always stay or support (in most cases) x16 lanes
Hope it helps and isn't too confusing

Thank you! I do understand now (along with some extra research done). I can run two of my GPU on my motherboard no problem because I have two x16 slots. I also have two x8 slots with will decrease the performance of the GPUs (according to my extra research ;D). If I try to run all at once, there will not be sufficient bandwidth.