Can run 4 x 1080 Ti per 1200W to be safe. 1080 Ti draw about 250-300 watts each depending on settings.

So if you go with 6 x additional 1200W PSU, thats 7200W / 26 ~ 276W per card.

To be safe, i say 1 x 750W for the system + 1 card and then another 3 x 1200W for the remaining 12 cards per rig. Then you have 300W per card easy.

The HP 1200W models are about 60Euro/each. https://www.ebay.com/sch/i.html?_sop=15&_from=R40&_sacat=0&_nkw=HP%201200W&rt=nc&LH_PrefLoc=5&_trksid=p2045573.m1684

Price compare

Standard ATX
6 x EVGA SuperNOVA G2 1600W (3 per system) ~ 2100 Euro (~350 each) = 2100 Euro total

vs

Build your own from server PSU
2 x EVGA SuperNOVA G2 750W (1 per system) ~ 200 Euro (~100 each)
6 x HP 1200W (3 per system)  ~ 360Euro (~60 each)
+ 6 x breakout boards ~120 euro (~20 each) (even cheaper i bought from Aliexpress)
+ cables 100 euro (even cheaper i bought from Aliexpress)

vs

Buy complete custom server psu kits
6 x 'GOLD RATED 1200 WATT 110-240V POWER SUPPLY KIT FOR GPU’S MINING' (includes board & cables) ~ 600 Euro (~100 each)
https://www.parallelminer.com/product/hp-gold-rated-1200-watt-110-240v-power-supply-kit-for-gpus-mining-zec-dash-eth/