Oh god no that’s exactly the wrong way to do supply dice. It would be at the end of an entire encounter.
You are right, that is the normal supply dice thing but there is an exception for ammo. I missed the last bit from Black Hack 2e usage die explanation after the column break and just registered the normal usage part.
LIMITED RESOURCES
AND THE USAGE DIE
When an item described has Ud and
then a number after its name, it is
considered to be a consumable, limited
item. The ‘Ud’ stands for Usage Die, the
number indicating what size die it is. For
example, oil (Ud6) has a Usage Die of 1d6.
When that item is used its Usage Die
should be rolled. If the result is 1-2 then
the Usage Die is downgraded to the next
smallest die in the following chain:
Ud20 > Ud12 > Ud10 > Ud8 > Ud6 > Ud4
When you roll a 1-2 on a d4 - the lowest
die in the chain - the item is expended,
and the Character has no more of it left.
After the column break it says:
USAGE DIE &
AMMUNITION
When tracking the Usage Die for
ammunition such as arrows and bullets,
roll the Ud after the combat is resolved.
Which changes the frequency significantly if adapting to D&D and its often multiple shots per round. A base d8 usage die set of arrows will last a minimum of 3 full encounters and an average of 9 or so? And no chance of running out in the middle of a fight.