I do not know where this “per bomber” thing is coming from…
But, SBR’s of whatever type can do a maximum of hte value of the territory being attacked per round (i.e. per nation’s move).
Ths UK can do a maximum of 16 IPCs to Germany in UK’s turn (10 to Germany, 6 to Southern, assuming no new IC’s for Germany).
And the USA can repeat that in THEIR turn.
And of coruse Russia could also… if somehow they were buying bombers and/or rockets.
So, the maximum SBR damage to Germany in 1 full turn is 48 IPC’s… 16 each from UK, USA, and USSR.
If you check all of the online dicey’s, you will find this is how it is rolled… and I believe Larry Harris Tournament Rules also clarify the SBR thing from the BS in the printed manual (which is TOTALLY wrong)