The scenery depends on what “look” you wish to achieve, the modelled location of your layout, and how much room does your track plan allow you to “play” withh, not only scenery but also industries. Which ideally should be factored into your “build plan.”
Off the cuff, make sure that you have enough clearance for your trains to run without snagging the scenery, this also applies to trackside buildings.
One thing I’ve noticed is because of lack of room, changes in scenery elevation are more vertical than in reality, that is unless your modelled area is the Rockies, or the Grand Canyon.
I’m not sure if the “New Forums” search function works better than it did on the “old forum” but there is enough good information there to thoroughly confuse you!
Here’s a link to just two previous threads…
https://forum.trains.com/t/layout-design/272503
Cheers, the Bear. 