How to get sub process value in main process.

Sub-processes can be tricky but work well when iterating through a list. When choosing to use 'multi-node instance' by iterating through a list, you can reference the list as a variable to pass into or save out to and Appian will smartly choose the correct index. If you know how many sub-processes you'll be creating, I recommend creating a variable of Integer which is a multiple and calling it Ids. You can use fn!enumerate() in a script task to quickly generate a list. Have the sub-process loop through this list and save your newly generated Id back to the list. 

That should work, if it does not, consider passing the variable by reference, https://docs.appian.com/suite/help/21.2/Sub-Process_Activity.html#passing-a-process-variable-as-a-reference which means you don't need to save back to the variable since updates automatically persist to the parent process.