I myself have run filers with over 2 million inodes. 6 million, 10 million, whatever... shouldn't be a problem, so long as you have enough disk space.
Obviously, when you deal with large metadata files performance can suffer, since the filer can no longer hold it all in memory and instead has to page portions in and out to disk. So naturally there is some performance penalty, but it's not like the thing is going to come to a grinding halt on you. If you do run out of CPU you can always move to more filers or bigger filers.
Nothing wrong with using Snapshots if you want them, too.
Bruce