10,000 amethyst darts give 210,000 Fletching XP total, since each set of 10 darts is 210 XP.

A quick breakdown:

  • 10 darts = 210 XP.
  • 1 dart = 21 XP.
  • 10,000 darts = 1,000 sets.
  • 1,000 sets × 210 XP = 210,000 XP.

Quick note

The source snippet also states amethyst darts require level 90 Fletching to make.

TL;DR: 10,000 amethyst darts = 210k Fletching XP.