I suppose I use 4 (on my Pre). I'd probably have an extra screen or two if I had cool widgets like a calendar or weather, but I suppose I'd also use fewer for apps if I had another mechanism to organize them.
Center is homescreen with clock and date, mostly for looks.
Left is calendar.
Right is widgets including weather, frequently used apps, and on/off toggles for most everything.
i have 3 set up but really only use 2... i downloaded folder organizer and have all my apps in folders on my first homescreen (probably my fav app of all time and free on 4shared.com), to the right i have a full page calendar, and to the left i have the today in history widget
I fill out all 7. My home screen is my weather widget with the clock. And under it is my most important apps (phone, emails) My other home screens are large widgets (4 of them) which fill up the whole screen, and two of them consist of apps.