When your session has more tracks in it than your monitor is wide or deep (depending on which window you are in) you need to manage you track display. Many people just scroll using the mouse wheel and modifiers. While there is nothing wrong with that, I prefer to use a variety of techniques to show and hide tracks according to what I am doing. There are lots of ways to do this but the one I usually introduce first is using groups and showing and hiding tracks based on their group membership.
To do this create mix or edit groups to associate tracks which belong together in some way. Tracks can be members of multiple groups and groups can be created and deleted at will so make as many as you like. To show only the tracks in a group Ctrl+Click (Start+Click on PC) on the group in the groups list. To display all tracks just Ctrl+Click on the All group, To add the tracks from a group to the currently showing tracks add Shift to Ctrl/Start and click. I usually create an "All Current" group if I have inactive or hidden tracks in a session.
There are other ways to manage tracks but I still find myself using this to complement other techniques in every session I work on.