From Chapter 12 of "Avaya Aura Application Enablement Services Overview and Specification", you can see that it is possible to register up to 4000 endpoints. For a recording application, these endpoints are usually managed over a single DMCC connection to the recording application. However, as DMCC is also used for softphones, it is reasonable to believe that you could use a single connection per endpoint.
There are some other limitations that I think you should investigate, for example:
o There is a limit to how fast you can register your endpoints. There is some information in the Overview on this, as well as some other Forum threads.
o The Communication Manager hardware has limitations too so you may need to invest in extra hardware. Again, some of the limitations are listed in the AE services Overview.