Should we use "Send Message Event" or "Sub-Process Activity"

This depends on a few factors not mentioned in your post. However, other than a few exceptions, a subprocess is a lower impact way to instantiate modular/reusable workflow components. How many child processes are you trying to start? Can you describe your usecase in more detail?

# Child Process Count is around 300. I want to Hold my process execution based on my CDT field and then it will create no of instances based on payload (Max 300).

# is it possible to change hold time on the fly. I searched and came to know that to edit constant on the fly I need to use plugin which I cant use.

Attached : process model.

Kindly give you valuable Advice..

If I understand your question correctly, you would like to delay each child process by a value stored in a cdt field.

If that's the case, you can pass in the number to the child process. Have the child flow directly into a timer event and use the number as the delay.

on the fly i want to change the delay count also...

How will the delay be changed- by a different user action?