The WBS-680 is designed to easily link two bases together using standard cables and minimal frequency coordination.
If one wanted to link more than two together it could be accomplished, but at this point the user will need to make a custom CAT-5 cable to support the linking of the WTA feature (Wireless Talk Around).  The current product offering only allows for two bases connecting together via a standard CAT-5 cable.  If one doesn't care or need this feature then it can be skipped.
The bigger issue is the coordination of frequencies when linking more than two bases together.  Ignoring any other frequencies in use (2-way radios, wireless mics and on-air DTV signals) the current product easily allows for two systems to link together using the factory preprogrammed frequency groups / channels using a single group with up to eight channels in that group. 
If one wants to use more than two bases together then some frequency coordination will need to be considered dependant on the location of use and any other wireless to be used in conjunction with the WBS product.  In some instances it would be possible to use other factory programmed groups in the same band or come up a custom group and channels using one of the last 12 customer frequency groups. 
If frequency real estate isn't a big issue then WBS systems on different frequency bands could be used to make systems of three base stations or larger, but again one has to coordinate frequencies in order to minimize intersystem frequency modulation, commonly referred to as IMD (inter modulation distortion) which is typically heard as a whining sound or high pitched buzz in the system depending on the offending frequencies involved. 
Successful rental houses and or experienced wireless intercom personnel can typically put together 4 or more WBS systems together for large award shows or events, but remember these folks have the benefit of years of experience and they typically use advanced frequency search programs and frequency sniffing equipment to make this happen successfully.