Technically speaking, there is a limit to the possibilities. The number of possibilities is astronomically high when you consider that one possibility is a level with 50 spikes and 100 goombas, while another is 49 spikes and 100 goombas, so on and so forth, but there is a definite limit of some kind.
Also, anybody who has played Little Big Planet can tell you that the vast majority of levels will not be winners. It is just not how this type of thing works. It especially doesn't help that, according to Siliconera, all of the review copies of Super Mario Maker are accessing different servers than the ones that will be used for the full release, so none of the levels you're seeing on youtube will be available when the game launches. This is a measure to prevent street date breakers from accessing online, of course. Completely understandable. It still means any good levels out there will fall into the nether when the game launches and the official servers go online.