Optix can use multiple gpus. Cycles may be limited to 4, but I am not 100% sure about that since I don't have a system with that many slots or pcie lanes. You won't get 2x performance from each gpu you add. You will get about 1.8 max with the exact same type of gpu. Less if you mix in a slower one. Any way you do it, you will see decreased render times, but never twice as fast from adding another card. Also keep in mind that many processors don't have enough Pcie lanes to run the cards at 16x when there is more than one of them. You will need a really good power supply as well. 

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