Solution:
As always, try to find a way of looking at the problem that makes the solution simple. In this problem, regardless of the current state of the puzzle, a move consists of combining two clusters into a single cluster. Therefore every move reduces the number of clusters by 1. Since we begin with 2000 clusters (the single pieces) and end with 1 cluster, we need
\[2000-1=\boxed{1999}\]
moves to complete the puzzle.