Based on the fact that the npc's speech for 1000 did not include any "wish you'd gotten more, but I'll make do with this" subtext, I think 2000 or 10,000 won't get you any better reward than 1000.
However, it is possible that the minimum required for the scroll reward is actually a lot less than 1000.
So I agree with Isaac that it would be best to try numbers less than 1000, rather than more. Using haha's idea of not-quite-turning-in the quest, go talk to Mr. Ku after every 100 roots or so, and see at what point his speech becomes the "hohoho" version (click "end chat" if it doesn't).
However, it is possible that the minimum required for the scroll reward is actually a lot less than 1000.
So I agree with Isaac that it would be best to try numbers less than 1000, rather than more. Using haha's idea of not-quite-turning-in the quest, go talk to Mr. Ku after every 100 roots or so, and see at what point his speech becomes the "hohoho" version (click "end chat" if it doesn't).