Estimates of the number of marchers is ranging from 30K or so to 2 million. There are the usual allegations of conspiracies to to inflate or diminish the total.
My personal feeling, given that it was shabbos and therefore I was only following the clean up after in real time, is that anything over 200K is extremely unlikely based on the performance of local mass transit system and the traffic. If we'd really had 2 million people show up in the city, it would have been much more disruptive to traffic and we would have seen a much higher spike in local business (which inevitably makes the local news). We also would have had noticable disruption in the overloaded cell network (as was discussed in preparation for the inauguration).
As is so often the case, the discernible facts on the ground leave room for a wide range of interpretations. Supporters view it as a great success, opponents as an epic fail. The real test, however, is what actual result is achieved. In this case, I do not think it added any new information for members of Congress or the Administration. Yes, getting even tens of thousands of people to one place to march is an impressive bit of work, even with corporate support and the support of Fox News. But these are the same people who showed up at town hall meetings during the summer. This doesn't do much to answer the "fringe v. mainstream" question -- especially as the marchers expressed only a general anger/opposition rather than a coherent mandate for anything.
There is a possibility that the march itself creates a separate movement that has legs in 2010. But that remains to be seen. Again, these are the folks who turned out in '08 to support Palin and oppose Obama even if they didn't care much for McCain or the GOP generally. As progressives hve discovered, opposition is easy, getting stuff done is hard.