As Ismail and Sjors say, testing your specific site is best. Then you will find out the limits of
your servers, bandwidth, infrastructure, and site's components.
In general terms, however, umbraco can handle very large sites indeed. Not only the number of nodes (200,000 for v2, I believe), but also lots of traffic. If you ever find your site getting slow, you can create a webfarm (not a webgarden) and use loadbalancing. Umbraco is ready for load balancing.
You might find this post and its comments interesting background:
http://objects.dk/2008/1/9/bigger-and-bigger-sites-choose-umbraco.aspxcheers,
doug.
MVP 2007-2009 - Official Umbraco Trainer for North America -
Percipient Studios