That is a little bit due to the shortness of the demo. I planned to have 120 regular levels in the final game. So it is now that if you use 1 extra jump it costs 12 ink and in the current level you can't collect some. That means for every level regularly beaten (and collecting all 24 ink bottles) you can beat two levels with one additional jump in each but then you have no more ink, so you got to beat the next without extra jumps or you go back and beat one again without extra jumps. The levels will get increasingly harder from the next world on, so you will eventually go back to older ones to get ink back for these harder ones (that is my hope at least). The first twenty are more to get the player comfortable with the game and its controls without the danger of getting stuck, so he can amount some ink for the first harder levels. There are also 4 bonus levels which get already a bit harder and there you can't use the ink exchange, but you have to find hidden keys in the levels to unlock them. They are more highscore oriented (time, jumps, killed rats, collected thingies). If you beat the 20 levels there is also an a knowledgement for beating the level without using the hover mechanic (star in the level select screen). Is the time ranking or getting all ink/coins interesting to you or are you just interested in getting through ?
(edit: in the shop you can get hints in which level the keys are hidden, if you find a key you don't have to end the level, you can just go to the level select screen through the pause menu)