Tracks are vertically expandable. While that's been implemented in different ways through the years, here's how it may appear in the current release.
Track_expand.mp4 There is no track limit if the Master's edition has been registered, but it's good practice to use as few as possible. Instead of creating a new track for a clip, try to use the same track as an earlier clip, as long as it doesn't conflict.
The volume of each clip can be adjusted independently, using the amplify or compressor or normalize effects. That may be your best bet.
Or you might try to export the sequence as a new audio file. Click the Sequences tab above the clip bin... 
If there is no Sequences tab, create a new blank sequence by clicking the + sign on the Sequence 1 tab above the timeline.
Right-click on the Sequence 1 thumbnail in the bin and Save Sequence As New Audio File. Now replace all audio tracks with that new, single audio clip.
The normalize effect can be applied, but results may not be as good as adjusting individual clips. For the options, read this.
Be sure to back up each operation, to revert if necessary. Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, the AS choice is available by clicking the caret to the right of the Save Project button on the toolbar. That way you're protected, and shouldn't lose anything.