Rita:
I don’t tumble yet, but have read a lot about this. I think
others will agreee that the smallest unit is appropriate for just
two pcs at a time. The Rio catalog used to (maybe still does)
give capacities in terms of number if pcs for the tumblers it
sells. A 3 qt will do 50 rings, so for just two about anything
will do, and will save you money on media. Stainless steel shot
is expensive and a 3 qt will take well over 10 lbs, I think, so
check the capacities for the amount of media required, too, they
are in the Rio catalog. Or call them, they consult on mass
finishing, maybe your other suppliers do, too.
Just be sure you’re not going to start doing production work any
time soon — if you did, you’d want something bigger. HTH