Normally, to find power supply size you add TDP of all components and add 20%. Google "TDP 7950" will get you the answer.

Anyway, it is 200W. You need 1000W for 5 GPU alone then, plus another 100-150W for the motherboard and processor. If you overclock you should plan for 225-250W per card even.

So, you need at least a 1100W power supply, but you should buy a 1200-1300W one for better results. Cheap ones are over-rated. Combining them is OK, but be mindful that you load them evenly.