It depends on which PFC card your 6500 has. If you have PFC 1 or 3, you can have up to 256 HSRP group ID which means that this card will support up to 256 well-known MAC addresses. Otherwise, with PFC2, it will only support 16 unique HSRP group ID. Consequently, it is a hardware limitation on the PFC2 card. You can have combined multiple VLANs to the same HSRP group ID (I don't like it) but it will work as long as you do not mix this up with other switch vendor products.
To read more about it, refer to this URL: http://www.cisco.com/en/US/customer/products/hw/switches/ps700/products_qanda_item09186a008011c6bb.shtml#q2.
NOTE: to find out which PFC card you have, issue "sh module" on your 6500 switch.
