I understand that this is a very open question, however;
From a practical point of view how many DIA database channels and Driver instances can a X4 support, based on its hardware.
It greatly depends on what data the channels contain, what types of driver instances you are using, what type of data processing or qualification, and where you are storing the results.
There have been tests using the Dia with large amounts of real world devices (128 XBee LTH/Wall Routers) that show the XBee radio and CPU resources being exhausted.
There have been tests using simulated devices that can create hundreds of device instances (with their corresponding channels) to the point of memory exhaustion.
Because this question is so open ended, I suggest defining a use case of what you want the Dia to do, and examine the resources that are required for each aspect.
For example I have an XBee sensor reporting every 15 minutes, that produces 2 channels, and requires 2 post processing transformations. Every hour I push the data to an external server using the tcpcsv presentation. If I desire to setup 8 XBee sensors per gateway, I need to quantify the resources used in a 1 sensor setup, define the resources that scale per device, or have a one time cost, and see if I find the CPX4 acceptably meets the requirements in terms of memory available, bandwidth, and processing time if I scale it up to 8 devices.