Two should be enough to do all attractions, under normal circumstances. Three is good to have a backup in case something isn’t available one day, to be able to take your time exploring, and spend more time with Power-Up Bands or interactive wands. Plus, repeating your fave experiences again, and trying more food!
If only doing 1 day, you’re going to have to pick your battles, and skip some stuff (shows and interactive experiences most likely) to get the greatest hits at least.