In some of the larger Virtex-6 FPGAs, there are up to 48 GTX transceivers. In version 1.3 of the GTX Transceiver Wizard, the maximum supported number of transceivers is 24. This Answer Record discusses a method for working around this limitation.
The GTX Transceiver Wizard generates location agnostic instantiations of the GTX. To generate wrappers for those GTX_QUADs that are not present in the Wizard, it is a simple matter to generate a wrapper for those transceivers that are included and tie them to the desired locations via the primary UCF in the design. The UCF is the only portion of the Wizard outputs that dictates the location of the GTX Transceivers in the design, so modifying only this portion allows the tools to incorporate the additional locations correctly.