I have no certain plans about how many levels I am going to create but my game sure has 5 worlds. Each world has 1 boss fight level like in SMB but instead of 1 miniboss level, I plan to include 1 special challenge in the middle of the way.
Also, I would like to have 1 secret world which has some extra levels including easter eggs. I want that it would be partly possible to enter the secret world from each regular world and that would reveal something special if you will be able to beat all secret levels from every regular world on. The reward may be one more extra level which maybe will include a secret boss. In addition to a secret world, I plan to include some secret ways so you would be able to skip a few regular worlds or even a special challenge. However, I am not sure if it is possible yet and I am not sure how it exactly will become.
I want to make something special but I don't want to spend too much time on this game so all other levels will be classical and simply made although I plan to make those look good. That is why I also started from the hardest and most important things on. But sure is that I plan to make the same amount of levels for each world and before and after the special level it would be normal to have at least 3 regular levels.
But my game is even not Beta yet so there will be a lot to do yet. My game reaches to Beta when all required worlds will be done and/or if the worlds and screens are fully illustrated.
Talking about the Power Plant also, water control has been now planned and constructed.