Hi Yevgeniy,
Good question and there’s a long answer…
Originally on the 3550 and some of the 6500 supervisors only 16 HSRP groups were allowed but this limitation applied only to a single interface. You could use the same group number on different interfaces without any issues. On newer platforms this 16 limit doesn’t apply anymore…the 3750 supports 32 groups I think and my 2800 router supports 255.
About authentication…originally the RFC 2338 standard described authentication but it was removed in RFC 3678 since it wasn’t secure. Not sure which RFC the Cisco implementation is based on but they probably added authentication to VRRP themselves.
Hope this helps!
Rene