create table test (
  company_id integer not null,
  client_id integer not null,
  client_status text
);


insert into test values
  -- company1
  (1, 1, 'y'),

  -- company2
  (2, 2, null),

  -- company3
  (3, 3, 'n'),

  -- company4
  (4, 4, 'y'),
  (4, 5, 'n'),

  -- company5
  (5, 6, null),
  (5, 7, 'n')
;

1
2
4
5

select company_id,
count(*) over (partition by company_id) as all_clients_count
from test
-- where all_clients_count != ... ?