Personally, if I'm printing a round of cards anyway, I don't mind doing 50 or more. More than 10 and I'm already in "production" mode, and I've made a few improvements to the darkroom to make this a little easier (Metrolux timer to prevent exposure drift, 4-up proofing easel, Rotatrim which I got for alt-process printing).
Objectively, before we had scheduled rounds and just printed whenever the inspiration struck, the Postcard Exchange seemed to grind to a halt when the list got to be over 40.
Tom--Postcard stock is indeed Portfolio RC paper with postcard printing on the back, and you could use 5x7". The only difference is that you won't be able to use the postcard postal rate.