Allow multiple monitors in Remote Desktop
Not version-specific
If using Windows Remote Desktop app and you use more than 1 monitor but only see 1 when you remote in, you can change it to allow all of your monitors to work with the remote session.
Estimation Desktop
Open Remote Desktop.

Click Show Options if not already expanded.

Click the Display tab and select the box "Use all my monitors for the remote session"

Connect to the remote session and now you should see the remote session with all your current monitors being used.