Can't speak for everyone, but I have...one. Just me. It's been two years and so far that's been enough.
I would say that when you have employees who need to be sending email to the outside world representing your company (to customers, suppliers, media, etc.) then you need to add more.
Hope that helps!
If you are not using "forwarding emails" this might help. I use them alot to specifiy functionality (cartsales, membersupport, etc) to forward to the person who actually fulfills the function.
Hope this helps,