With 400A of 208v, you can run 80 S9 13.5, should be able to load panel with 40 breakers, than on a few of them just run 1 S9 and 2 L3. Obviously just watch the panel when loading up, transformer with poor harmonics will tend to draw lower voltage increasing amperage.
80 s9 running at 6.5-7 amps each would be well over 500 amps. How exactly did you make your math?
400 amp panel @ 80% capacity is 320 amps of usable power. S9s drawing 7 amps each would mean you could run 45 total. You would probably want to run less because you need to leave power for ventilation and other things.
I also run 3 phase 208. My setup is like this.
I run 3 phase all the way to the PDU. To balance power I run them as follows....
Miner layout per PDU:
Phase A Phase B Phase C
PDU 1: 2 1 1
PDU 2: 1 2 1
PDU 3: 1 1 2
I run 9 breakers per sub panel and balance the 3 phases to the panel this way leaving the 10th breaker for ventilation. The draw across the 3 phases as measured at the subpanel stays within a few amps of each other. (Not all machines pull the exact same amount of power, I have some S9s pulling 1380w, some pulling nearly 1500w depending on batch and PSU used.)