I have always followed the one extra paddle for every two canoes rule.  In 19 years of trips, we have only needed the extra paddle once.  

Ironically, it was this year when I stepped and broke a nice wooden paddle.  The humerous part is that we were day tripping with two canoes and forgot the extra paddle.  My 10 year old son enjoyed the long row home from the bow as I did all of the paddling while he got to watch the sunset.  It was interesting to note that paddling my SR17 solo from the stern I was able to keep pace with my brother and my son's 10 year old friend with no problem (they were in a Old Town Guide canoe that goes ~ 75 lbs).