homer's facts are not quite correct. Win 98SE could support up to 8 monitors. Numerous sources say Win XP Pro can support up to 9 monitors. But, in fact, as many as 16 monitors have been run with Win XP and with 4 PCI Matrox Millennium G200 quad-monitor video cards.
Also, running 8 monitors can done other ways than with 2-4 port video cards. In other words, more than 2 video cards could be used to accomplish it. But, let's talk about a more realistic scenario, the 4 or 5 monitors you mentioned. 5 monitors could be supported with 3 video cards, with at least of them supporting two monitors. You would not need a quad-monitor video card; cards supporting two monitors would suffice.
I've had 3 CRT monitors connected to my computer, using 3 video cards, 1-AGP and 2-PCI. I currently have two CRT monitors, both attached to my AGP card, with a nVidia GeForce4 Ti4200 GPU, and with VGA, TV-out, and DVI connectors. The primary monitor is connected to the VGA port; the seondary, to the DVI port via a DVI-to-VGA adapter, since that monitor also requires a VGA connection.
I could install the two PCI cards again and support 4 monitors. Or, I could get a single PCI card that supports two monitors. So, you can see there are a variety of possible configurations. If you'd like to see a database of actual configurations, see it
here.