If we're not factoring in difficulty of collecting stuff, then yeah, I would say Yoshi's Woolly World wins out for collectibles (at the time this was asked, which was before Super Mario Odyssey). Given that there are 6 worlds, 8 levels + 1 secret level per a world + the final bonus level, that's 55 levels altogether. There are 5 yarn balls, 5 flowers, 20 stamp coins and 20 hearts that need to be collected per a level, bringing the total to 50 collectibles per a level. Altogether that makes it 2750 collectibles in the game. I can't think of any Mario game (at the time this was asked) that would be able to top this.
