Get into Warcraft III World Editor and open your map. Click Advanced then Gameplay Constants. Up the top click Use Custom Gameplay Constants then scrolll down to Hero Maximum Level. The Maximum Level a Hero can go is 10,000. You can only change it if you have clicked Use Custom Gameplay Constants.
I don't think that 10 000 is the maximum. If you hold down shift when doubble clicking on the field you can go higher. But why would you want to do it?