Getting a Normal ECE Site to Scale
To start with the easiest; getting a normal, high traffic ECE site to scale is "not a problem" as it's "only" a matter of caching the content right.
Roughly speaking, you need to make sure to have tuned the ECE
caches (see the "Escenic Content Engine Admin Guide" for this),
run a distributed memory cache (
ease the load on the databases (again, see the "Escenic Content
Engine Admin Guide" for this) and a well configured cache server
in front of the application servers, such as Akamai, Squid or Varnish.
The sizing of the server rig should be in the lines of 6-8 application servers, 2 DB servers, HA solutions (i.e. by using HA proxy and virtual IPs) for file system & RMI-hub and 2-4 cache servers (multiplied with two if you are using Squid 2.x).