A certain number of rides, shops, restaurants and decor is required to max out your Krust-O-Meter (for more info on what a Krust-O-Meter is, look here).
As you can see, to get the full 5 stars in all the ratings, and thus 5 stars in your total Krust-O-Meter, you need 7 rides, a whole bunch of decorations, 18 restaurants and food stalls, and 18 stores. (It’s nice that there are exactly 7 rides which don’t require donuts). All of the buildings and rides add 10 points to either Nausea, Grub or Merch ratings, but the decorations point values vary.
These requirements don’t change based on your game level (for now, though if they add more rides/buildings/decorations I expect that to change). So someone at level 20 needs the same amount of stuff to max the Krust-O-Meter as someone at level 32.