Here's a link to the "End-of-Sale and End-of-Life Announcement for the Cisco 3600 Series Routers"
http://www.cisco.com/warp/customer/cc/pd/rt/3600/prodlit/2114_pp.pdf This document has the milestones listed for the 3660 series.
The 3745 is shown as the recommended replacement for the 3660 Enterprise routers. The 3745 has 4 network module slots, so with 4 NM-32A modules, you'd have a total of 128 async ports. The unit also has 3 WIC/VWIC slots.