There are 43 passengers to be found. You can find the list of them at the bottom center of your inventory, found passengers will be crossed out. Thier locations can be found at
https://theforestmap.com/wiki/passengermanifest/ just check in-game whoever you've not found yet and click the matching name on the site it will mark thier location on the map. Keep in mind that seats 6B and 6A are you and your son Timmy, so they do not count towards the achievement.