On the catalyst 6500 switches, the maximum number of HSRP group IDs are limited to 16 in the Supervisor Engine 2-based systems. This limit is 256 in all other older and newer systems. You receive the below message when you try to create a new standby group after this limit.
More than 16 standby groups not supported in this platform.
Cisco says that this is a per-interface limit so you can use the same ID in several interfaces.
You can find the details on “HSRP Group Limitation on Catalyst 6500/6000 Series Switches Frequently Asked Questions”