The 21 mm is the width of the rim. This number has little to do with the size of the inner tube. You'll want to choose a tube size appropriate to your tire size, i.e. in your case, 32 mm. The advice above to get a patch kit is excellent. Patching is reliable and quick; much quicker than trying to find a new tube.
From the Trek website:
Rim: Bontrager Affinity Disc, Tubeless Ready, 24-hole, 21mm width, presta valve
Tire: Bontrager R1 Hard-Case Lite, wire bead, 60 tpi, 700x32c
Max tire size 38c