There is no set amount of either needed.
Best advice for scenery would be the more ruins and other scatter terrain the better - you don't want too much open space providing lots of areas with long uninterrupted line of sight to stop missile troops and some wizards' spells dominating the game.
For wandering encounters the answer is whatever you've got or fancy getting use them, even if it's 'counts-as', and what you don't have don't worry about.if you roll for something on the random encounters table that you don't have just roll again, or even better make your own table based on what you have.