The 1080 ti needs *2* PCI-E connectors (one 6-pin one per card - there are exactly ZERO power supplies in the under-1500 watt range with enough PCI-E connectors to run 6 of those cards (and I think you have to go to 1600 watt power supplies to get to 12+ connectors) - which doesn't factor in whatever you have to use to power your risers.
Yep, that is true, we had to get a 1600 watt PSU in order to power our rig with 6 x GTX 1080 Ti. Currently the rig only uses ~1027 watts at the wall (power limit at 62% for best efficiency), but in order to have sufficient cables/connectors that was the best single PSU option. Ideally a server PSU would be used to power all or most of the GPUs, but they are too loud and high pitched for me to tolerate in a living space.