when a motherboard has 12 slot PCIe then all slot using 12 GPU on the casing. I saw that casing, 6 GPU placed on the 1st floor below and another 6 above 2nd floors.
Normally this is for 10 cards with 10 stage (you could see another image on that store). From airflow sides, this Rig Case is suitable for single fan cards with one direction airflow, backside fan will push hot air, and the front fan will drawing hot air. But yes, it spaces to small for troubleshooting.