I might not be clear about the question. But google can index unlimited no. of domains. But it shows up only 1000 results per keyword.
I think no limit too. Look at digg.com : http://www.google.com.br/search?hl=pt-BR&q=site:digg.com&meta=&aq=f&oq= Over 11millions
I think there would be no problem because Google is regularly spending huge amounts of money over purchasing new servers and powerful online storage spaces because quality content can always get crawled and indexed with no limitations in the future.
I don't believe there is a limit. Though if you do have that much, make sure your site is spider-friendly. Sites like Myspace and Digg have tons. Google also has some of the best web-techs and programmers, I think they can make sure the servers can index that many.
I think its unlimited its because my past client has almost 10,000 pages but some is not yet indexed.