Flame Arrows says that:
The spell's magic ends on the piece of ammunition when it hits or misses, and the spell ends when twelve pieces of ammunition have been drawn from the quiver.
Does the Flame Arrows' magic end on the pieces of ammunition when the spell ends, even though the spell does not say so? If so, then Flame Arrows effectively only enchants 11 pieces of ammunition because the magic ends on the 12th piece before you can fire it.
On the other hand, if Flame Arrows' magic does not end on the piece of ammunition when the spell ends, then nothing is stopping me from stockpiling several dozen enchanted arrows days in advance.
The first reading seem weird and disappointing, while the second one is even weirder, but I do not see an alternative. If either reading were the intent, I would expect the Flame Arrows to be more explicit...