Choosing 23 of them at random to be visible will not guarantee a valid puzzle. I am not trying to say not do it, I'm trying to save you a lot of headache trying to make a very advanced game. If this is your first game, I would suggest starting with simpler puzzles, or forgo the random Sudoku generator in game and include a set number of pre-fabricated Sudoku puzzles.
Many crossword apps push new crosswords out in weekly updates rather than trying to include a large database or a generator. You could follow the same model.