I'm curious as well, especially since I already have a pair.
Unfortunately, it looks like they support a rather meager 64 tunnels. I'm not sure if that's for IKE sessions or IPSec SAs, but I am assuming the latter.
Here's my source for the tunnel limit:
http://www.cisco.com/c/en/us/td/docs/routers/asr920/configuration/guide/sec_vpn/sec-ipsec-xe-3s-book-920/overview_of_ipsec.html
++I;