With ASR you need a special link to exchange flows information.
This link goes from the master to the backup.
Finally, there can be only one link (and a backup link).
So, you can have only 1 backup device for ASR.
For VRRP, the protocol allows more than 1 backup (no limit actually).
But we only tested with 2 devices (1 master and 1 backup) so we officially only supports 1 backup.
Gilles.