Complex sentence: Despite the "-teen" ending, which would seem to indicate that it lies between 12 and 20, umpteen can be used in ways implying it is much larger than that--if it ever could be pinned down.
Split into simpler sentences: Despite the "-teen" ending, which would seem to indicate that it lies between 12 and 20. Umpteen can be used in ways implying it is much larger than that--if it ever could be pinned down.