I routinely have more than 30 open... in each of 2-4 different browsers (Chrome, FF, Opera, IE).
So a total of at least ~100 quite often. I find separating my browsing "subjects" by actually using different browsers works pretty well. Each one remembers where it left off when I reboot, too.
Speaking of tab organization, has anyone played with the new tab stacking in Opera 11 yet? I haven't had much fall for it so far...