I think the maximum is 4 for one GPU, but I really do not know for sure. I have never heard of anyone exceeding 4. If you want to exceed 4 displays, then you might need to 2 GPUs not in crossfire mode, and I am not sure if 2 GPUs will coordinate and accommodate 4 x 2 distinct displays.
If you don't have 2 dedicated GPUs, but have a dedicated GPU and iGPU, you could try setting up 4 displays to the 1 dedicated GPU and 1 display on the iGPU, then verify that 5 displays work.