If you underclock them with
Braiins OS you can even add more of them. By raising efficiency you can easily lower that consumption to under 820w each (@10T)... You could put 48 S9s, maybe some more if a few have damaged boards.
480THs vs 390THs using the same 40kW...
Make sure you check more sellers, i have seen S9s at $150 each...
PLUS, under-clocking tends to make them run cooler and last longer.
This is good idea , but as far as I can guess OP is really concerned with ROI or ROR so I did want to do some calculations, just take a look at the table below I tried to put some numbers to make it more understandable.
Note that the green columns are same for both quantity so no matter what you put result will be same, as well as BTC price ( i assumed 5950 $ you can put whatever you want ).
so u can see 29 pcs have better ROR.
http://i67.tinypic.com/2rwnuxk.jpg