From my research, you can't. The August blockchain driver will not run more than 8 GPUs. Someone on this thread explains it...
https://www.reddit.com/r/EtherMining/comments/7guky1/can_you_run_more_than_8_amd_cards_with_blockchain/AMD updated the kernel in 17.10+ to support > 8 GPUs on Windows10. Ethash/DaggerHashimoto mining efficiency is incorporated into the 17.11+ drivers, but not for CryptoNight or NeoScrypt.
If you're mining Ethash/DaggerHashimoto , then you can move to 17.10+
If you're mining CryptoNight/NeoScrypt, then stick with Aug23 blockchain drivers