On blockchain drivers, 8GB of system RAM, 48GB of Virtual mem.  I can enable HBCC on 4/6 Vega 64's without issue.  I can in rare cases after a full clean reinstall get 5/6 cards to work.  RAM and VMEM is not the issue here.  Shot my Virtual mem up to 96GB and my system mem up to 32GB.  Same outcome. 4/6 can HBCC, 2 cannot.