As of the first Breakout wave, Furno has been released as a constraction figure more times than any character from BIONICLE. However, that's not counting the BIONICLE sets with minifigures. In that regard, I believe Hewkii was the winner with ten appearances:
1. Huki (2001)
2. Hewkii (2003)
3. Toa Hewkii (2006)
4. Lava Chamber Gate (as a minifigure, 2006)
5. Piraka Stronghold (as a minifigure, 2006)
6. Race for the Mask of Life (as a minifigure, 2006)
7. Toa Hewkii Mahri (2007)
8. Barraki Deepsea Patrol (as a minifigure, 2007)
9. Toa Undersea Attack (as a minifigure, wearing a Volitak for some reason, 2007)
10. Toa Terrain Crawler (as a minifigure, 2007)
Taking that into consideration, Furno has a ways to go before he dethronres the true King of Being Released Over And Over Again.
1. Huki (2001)
2. Hewkii (2003)
3. Toa Hewkii (2006)
4. Lava Chamber Gate (as a minifigure, 2006)
5. Piraka Stronghold (as a minifigure, 2006)
6. Race for the Mask of Life (as a minifigure, 2006)
7. Toa Hewkii Mahri (2007)
8. Barraki Deepsea Patrol (as a minifigure, 2007)
9. Toa Undersea Attack (as a minifigure, wearing a Volitak for some reason, 2007)
10. Toa Terrain Crawler (as a minifigure, 2007)
Taking that into consideration, Furno has a ways to go before he dethronres the true King of Being Released Over And Over Again.