Custom packaging is manufactured to your specification rather than pulled off a shelf, so the quantity is part of what gets built. That makes the first number more consequential than it would be for a stocked item: it is not just how much you buy, it is how much gets made.
The two failure modes are symmetrical and both expensive. Order too little and you are back for a second run before you planned to be, which may not match the first if anything has changed in the meantime. Order far too much and you have paid for packaging that then occupies your stockroom, sometimes for longer than the design stays current.
The useful approach is to start from how fast you actually use the packaging, not from a round number that feels safe.