I think 2811 can handle this task
2811 supports up to 1500 VPN tunnels with the AIM-EPII-PLUS Module
http://www.cisco.com/en/US/products/ps5881/index.html
So no problem with 165 VPNs...
If you speaking about huge traffic volume - you should focus on speed of Internet connectivity in head office - If you have 10 Mbit line for head office you get only 60Kbit per tunnel (10Mbit/165)
M.
Hope that helps rate if it does