Question is a 850 WATT PSU enough for a single rtx 2080 ti

That PSU is nice.

You haven't listed your entire system but
2080ti = 300W max
Cpu = 150W
Other stuff - 30 watts
Try not to run PSU's etc over 70% power
so if the total is about 500W then anything from a 650W will be good
yeah sorry heres the list

asus rog maximus ix hero
intel core i7 6700k
corsair vengeance LPX 16gb 3000hz
evga supernova g3 80 plus gold fully modular
western digital 1tb 7200 rpm
samsung evo 500gb
gtx 980 ftw acx 2.0 4gb