Either use several graphics cards. Or use a graphics card supporting 6 monitors.

For example, PowerColor ATI Radeon HD5870 2 GB DDR5 6x Mini-DisplayPort PCI-Express Video Card AX5870 2GBD5-M6D can support up to 6 displays using a single enabled ATI Radeon graphics card with Windows Vista or Windows 7 operating systems. The number of displays may vary by board design and you should confirm exact specifications with the applicable manufacturer before purchase.
Also note that to enable more than two displays, additional panels with native DisplayPort connectors, and/or DisplayPort compliant active adapters to convert your monitor's native input to your cards DisplayPort or Mini-DisplayPort connector(s), are required.

Systems using multiple ATI Radeon graphics cards can support a maximum of 8 displays (total across all cards in system) with a maximum 6 of those displays being used together in a display group (also known as a single large surface mode).