If he kills one tenth transformation beast with each arrow of 1,500, he will gain 150,000 points. That isn't so in reality, though.
In reality, he misses 10% of his shots and 20% of the ones that hit that don't kill the beasts immediately. So only 70% of his shots can grant him kills worth 100 points each.
70% of 1,500 arrows is still a lot. If 70% of that many arrows grants him 100 points, he will still gain 105,000.
So the limit is not caused by a lack of arrows. The thing limiting him is that there are no more beasts around to kill.
There is a limit to the number of beasts in the waters around each island. Right from the moment he reached 5,000 points, he had begun to be constrained by this limit.
After killing 50 beasts at the tenth transformation, he gained 5,000 points. Unfortunately, the population of such beasts began to dwindle at that point.
