I had 1080ti above a 980 direct on mobo in a closed case, there was basically no clearance between them, air flow sucked and they both overheated. The ti's are quite fat, should use risers if you have more than one or have at least 2-3cm clearance between cards.
Most i've seen the ti draw was 300w, I bet you can run 4-5 ti's on 1200w psu.