Hi,
Honestly, I don't have a magic number for you on my 6 months experience with MT on a similar scenario from you we were able to push at least 1K user and counting from two CCR1036 one is dedicated as PPPoE server and the other is doing plain NAT. On the PPPoE server we disable connection tracking and adjust the firewall rule to a bare minimum.
If you have multiple CCR2004 at your own disposal try to do the same, because doing NAT and PPPoE service in the same box is a recipe for disaster when your are reaching 800+ active pppoe connection at least from what we saw in 1036. I think CCR2004 is design with ROS7 in mind may yield to a better result if you have time you can also experiment on that, good luck
Hiya on NAT u mean CGNAT right? its wise to separate both... Link ISP from BGP <-------> CGNAT1036 <------------> pppoe-server 1036