I know this is an old comment, but I thought I'd respond anyway. When I opened my portfolio towards the start of the game, there were eight total pictures and only three of them were not blank. The funny thing is, at that point I had only drawn three pictures! However multiple times (five, if i had to guess) I fell into the void or accidentally entered a door with nothing drawn, which prompted Professor Qwertz to say it wasn't finished and to come back when I was done. My theory is that the game catalogues the unfinished drawings as well as the finished. I would recommend going back through your portfolio if you still have it and checking to see if there are the 50 total finished drawings.