I've been curious about this for a while now, so thought I'd just ask here.
We could all take guesses (mine would be somewhere between 1,000 - 10,000), but I suspect someone at Ariba could find out quickly by simply checking how many suppliers are configured for punchout on the Ariba Network (on the production side).
That'd be a good indication, although not complete since there are lots of other systems now that support punchout as well. And of course this doesn't include OCI RoundTrip.
I don't see any harm in letting us know, in fact I think the number would be a testament to the popularity of punchout.