In Ansible, add an element to every dict in a list of dicts, but multiply it by an existing entry

I have a list of dicts:

vars:
  sites:
    - name: "Tampa"
      site_number: 0
    - name: "Miami"
      site_number: 1
    - name: "Daytona"
      site_number: 2

I want to add an entry to every dict in this list, but multiply it by a value. I have tried the following:

- set_fact:
    sites: >-
      {{
        sites | map('combine', {
          'site_subnet': '172.16.0.' + (item.site_number*8)|string + '/29',
          'test1': item.site_number
        }) | list
      }}
  loop: "{{ sites }}"
- debug: var=sites

Now, the resultant output shows that sites[n].test1 is always 2, and of course, the site_subnet is always 172.16.0.16/29. What is the malfunction with my code?

Ansible is generally not a great tool for modifying data structures. You can get what you want by creating a new variable, like this:

- hosts: localhost
  gather_facts: false
  vars:
    sites:
      - name: "Tampa"
        site_number: 0
      - name: "Miami"
        site_number: 1
      - name: "Daytona"
        site_number: 2
  tasks:
    - set_fact:
        new_sites: >-
          {{
            new_sites + [item | combine({
              'site_subnet': '172.16.0.' ~ (item.site_number*8) ~ '/29',
              'test1': item.site_number
            })]
          }}
      loop: "{{ sites }}"
      vars:
        new_sites: []

    - debug:
        var: new_sites

This results in the following output (which is what I think you want):

TASK [debug] *************************************************************************************************************************************************************
ok: [localhost] => {
    "new_sites": [
        {
            "name": "Tampa",
            "site_number": 0,
            "site_subnet": "172.16.0.0/29",
            "test1": 0
        },
        {
            "name": "Miami",
            "site_number": 1,
            "site_subnet": "172.16.0.8/29",
            "test1": 1
        },
        {
            "name": "Daytona",
            "site_number": 2,
            "site_subnet": "172.16.0.16/29",
            "test1": 2
        }
    ]
}

For any sort of complex logic, I always find it more managable to use a custom filter instead. I would drop the following into filter_plugins/filters.py:

def filter_update_sites(sites):
    return [
        site | {
            "site_subnet": f"172.16.0.{site['site_number']*8}/29"
        }
        for site in sites
    ]


class FilterModule:
    def filters(self):
        return {
            "update_sites": filter_update_sites,
        }

And then write the playbook like this:

- hosts: localhost
  gather_facts: false
  vars:
    sites:
      - name: "Tampa"
        site_number: 0
      - name: "Miami"
        site_number: 1
      - name: "Daytona"
        site_number: 2
  tasks:
    - set_fact:
        new_sites: "{{ sites | update_sites }}"

    - debug:
        var: new_sites

Which produces:

TASK [debug] *************************************************************************************************************************************************************
ok: [localhost] => {
    "new_sites": [
        {
            "name": "Tampa",
            "site_number": 0,
            "site_subnet": "172.16.0.0/29"
        },
        {
            "name": "Miami",
            "site_number": 1,
            "site_subnet": "172.16.0.8/29"
        },
        {
            "name": "Daytona",
            "site_number": 2,
            "site_subnet": "172.16.0.16/29"
        }
    ]
}