You must SEE every pokemon in the Sinnoh Dex. That doesn't means catch anything, nor is it all 492 pokemon. There are 210 pokemon in the Sinnoh Dex, and that's what you need to see to unlock the resort and survival areas, and also expand to the national Dex.
If you battled every trainer you saw throughout the game and didn't use repels forever, you should have the Dex complete. If you missed any, you can google to find where they are located.
If you battled every trainer you saw throughout the game and didn't use repels forever, you should have the Dex complete. If you missed any, you can google to find where they are located.