6 bulkhead is nothing. Since IJN armour is worthless I using bulkhead + bulge. With good
SD you better withstand hits. Of course you wont survive block shot or constant pouding
from 10 ships but you'll survive much longer in battle when dueling. I say ~10-15 bulkhead
for Mogami (Myoko) is quite enough (ship defense strenght ~13). I noticed also that BH
helps when you hit islands (in Op.C.), damage is slower (especially if you put 20 or more
BH). For loosing sailors I dont know. I loose rookies quite often but experts veeeeeery
rarely.

I have 6 bulkhead on my mogami and I swear I lose more sailors now than before I put it on! :x

My excperience is yes. On my DD i put about 5 and i loose much less men then prior.
Since i only have a DD i can`t tell how much you need for a BB though...