My coworker bought a bunch of chains from the same guy for his husky.Wow, this is enough chains for the rest of my life! 12 chains! I can just see my wife rolling her eyes!
http://www.ebay.com/itm/12-Oregon-S...368?pt=LH_DefaultDomain_0&hash=item3a7a1c12e8
I doubt the saw will last for 20chains but you can always sell them at $5 a pop if it ever gives up the ghost.
I keep 5 chains sharp for my 20" bar and 2 for the 28". That way when a scrounge comes up or I knick one it doesn't cut into the woodcutting time.
Just sharpen all of the used ones at once in my bench vise with the viking guide on a rainy day.