Fewer than most people imagine.
Consider this acid test suite of Google web applications that I routinely use:
- Gmail with Google Video Chat
- Google Apps: Documents, Spreadsheet
- Blogger and BlotThis!
- Google Sites
- Google Calendar
- Google Page Creator*
- Firefox (XP and OS X)
- Safari (OS X)
- Camino (OS X)
- Chrome (XP)
Just Firefox. Even today's Chrome has obvious bugs, such as omitting the final character from a link created against an existing text string.
I assume IE 7 works pretty well too, but I can't speak to that. I'm surprised Chrome still doesn't work as well as FF against Google's own properties.
It's still incredibly hard to deliver full function "web 2.0" apps against more than one browser.
* On death row and supposedly due for replacement by Google Sites, but that seems to be on hold.
** I use IE 7 regularly against Sharepoint at work, but nowhere else