ansible : how to use the variable ${item} from with_items in notify?

Handlers are just 'flagged' for execution once whatever (itemized sub-)task requests it (had the changed: yes in its result). At that time handlers are just like a next regular tasks, and don't know about the itemized loop.

A possible solution is not with a handler but with an extratask + conditional

Something like

- hosts: all 
  gather_facts: false
  tasks:
  - action: shell echo {{item}}
    with_items:
    - 1 
    - 2 
    - 3 
    - 4 
    - 5 
    register: task
  - debug: msg="{{item.item}}"
    with_items: task.results
    when: item.changed == True