teledb=# set max_parallel_workers_per_gather to 0;
SET
Time: 0.214 ms
teledb=# select count(1) from t_count;
  count   
----------
 20000000
(1 row)

Time: 2700.569 ms (00:02.701)
teledb=# set max_parallel_workers_per_gather to 2;
SET
Time: 0.269 ms
teledb=# select count(1) from t_count;
  count   
----------
 20000000
(1 row)

Time: 1867.719 ms (00:01.868)