I for one have very much enjoyed taking part in the last exchange, and will undoubtedly take part in the next (I've already started trying to pick my negative...).
For me printing 35 postcards was not a problem and I'd gladly print many more if necessary. I'd just split it into several darkroom sessions.
I do think Nige makes a good point about accommodating people who don't have the time or resources to print a very large number of cards though. One of the nice things about these exchanges is how inclusive they are.