Does PhoenixMiner have a GPU max limit? I have a RX580 rig that I cant get to run more than 8 GPU's. It will either immediately shutdown after you start the miner and close the window or it will fail to load the dag file and restart the miner after showing a specific GPU error. All RX 580 cards are 8gb and the O/S is Windows 7.
Miner supports as many gpus as you can get working. Ive got some 13 gpus rigs on it no issue.
I will add im on linux so maybe a windows (or windows 7) thing.