You used up all 48? I thought I was the only one who ever did that!

Far as I know, this would require hard writing the multiple tracks to a stereo wave file, then importing that stereo wave file back in after removing the originals to free up space.

Select the desired tracks
Solo them
Use Render-Merge audio and VSTi tracks to stereo wave file

Delete the no longer needed tracks (after backing up the original song)
Import the mixed stereo wave file.

If you have a couple free tracks left you may be able to experiment with highlighting the tracks, then using right-click
Tracks-Merge
this lets you merge some tracks together on another track, again allowing you to delete the originals if satisfied with the result.

In this situation, I'd want to make sure I had a copy of the original tracks by using Save As so I could open that file and have access to the original tracks later if needed.
Normally I count on RB's backup system to handle this, but in this case I'd want to make sure I had a copy of the original tracks somewhere before deleting anything.
Disk space is cheap compared to losing a performance.

Make your sound your own!
.. I do not work here, but the benefits are still awesome