As an alternative data-point:
My firefox session is persistent (tab are restored when I open firefox again). Apparently, I'm around 500 right now, which is a bit above average for me (I usually stay around 300).
Firefox's lazy loading is very good, which means I only have 10 to 100 tabs that are actually loaded. Firefox is still fast in these conditions. Since I use tree-style-tabs, most of the tabs are neatly sorted in trees by topic.