To help you get the right product we would ask you these questions: 

1) What host PC or laptop are you using for expansion (manufacturer and model #)?
2) How many host slots do you want to use with the expansion system?
3) What type of slots are available for our host cards (for example one PCIe 3.0 x16 and one PCIe 2.0 x8)
4) How may GPUs do you want in the expansion system and what models do you plan to use?
5) What are the power requirements of the GPUs?