I don't see a single 1500 watt power supply handling more than about 5 1080 ti at one time reliably (6 if they are FE cards *maybe* but that's pushing the supply to the limit).
You MIGHT be able to push it to 6 (aftermarket) or 7 (FE) with a fairly hefty lowered TDP limit on all the cards but keep in mind that the cards run at 100% TDP and normal BIOS voltage when the rig first boots up, for a few secs at least, 'till whatever you're using to SET the lowered TDP kicks in - definitely put a sleep of at least 10 and preferably 20-30 seconds on the first line of your "start up the miner .bat file" to give everything else time to get going and initiated.
1500 + 1200 should handle 8 comfortably - 1500 for 5 cards, 1200 for the other 3 cards + the rest of the system.
Don't believe the "1080 Ti is a 250 watt TDP card" information - a LOT of the aftermarket cards set the TDP quite a bit higher to give them the OVERHEAD to be able to set those high "factory overclocks", and even on GAMING loads (which don't load up a GPU nearly as consistantly as mining does) a lot of review sites quote some of the 1080 ti cards as being MEASURED at 290 watts or so usage.
listen to QuintLeo - I have 6 that's been connected to my killowatt for the past 3 days and the draw from the wall is 1310 watts - that's at 70% TDP so at 100%... good luck!