Insert elements postgresql with maximum row

I am trying to create a insert script with postgres,

To insert into a table a maximum 1 element per person,

For example, I got this query down, but I would like to create a logic that if I already inserted a file to a person, the next file should go to the next person of that country

I'm so stucked with this situation, I've never seen something like this before, a script that insert just one row and then inserts to the next "person".

What is happening now is that I'm inserting 3 files to a person, and the other person from the same country doenst receive a file.

WITH get_files AS (
	SELECT
		type,
		country
	FROM files
	WHERE title IS NOT NULL
)
INSERT INTO history_person_file (
	name,
	document_type,
	age
)
SELECT
	p.name,
	f.type,
	p.age
FROM person p
LEFT JOIN get_files f
ON p.country = f.country
ON conflict (name) do nothing;

Perform the restriction logic in your query. Once you have that right, insert the result into your history table:

with num_docs as (
 select type, country, 
        row_number() over (partition by country 
                               order by title) as rn
   from files
  where title is not null
), num_person as (
 select name, age, country,
        row_number() over (partition by country
                               order by name) as rn
   from person
)
select p.country, p.name, d.country, d.type, d.age
  from num_docs d
       full join num_person p 
         on (p.country, p.rn) = (d.country, d.rn);

I used a full join here so you can review the results before loading to your history table.

Not sure but maybe this will work

WITH RankedPersons AS (
SELECT
    p.name,
    p.age,
    p.country,
    ROW_NUMBER() OVER (PARTITION BY p.country ORDER BY f.title IS NULL) AS 
row_num
FROM person p
LEFT JOIN files f ON p.country = f.country AND f.title IS NOT NULL
),
NextAvailableFile AS (
SELECT
    f.type,
    f.country
FROM files f
LEFT JOIN RankedPersons p ON f.country = p.country AND p.row_num = 1
WHERE p.row_num IS NULL
LIMIT 1
)
INSERT INTO person_file (name, document_type, age)
SELECT
p.name,
COALESCE(naf.type, 'default_document_type'),
p.age
FROM RankedPersons p
LEFT JOIN NextAvailableFile naf ON p.country = naf.country
WHERE p.row_num = 1
ON CONFLICT (name) DO NOTHING;