question about parallel. i read document and i can't solve it. provide the solution please.

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/18/when-can-parallel-query-be-used.html
Description:

I'm inquiring because I have some problem getting parallel processing to run
effectively in PostgreSQL.

My current parameter settings.
max_parallel_apply_workers_per_subscription = 2
max_parallel_maintenance_workers = 8
max_parallel_workers = 8
max_parallel_workers_per_gather = 4
max_worker_processes = 8

We are using a 4core CPU. When I set the table's parallel_workers to 8 or
use a hint like /* + Parallel (a 8) */, the EXPLAIN ANALYZE output shows: ->
Gather immediately followed by Workers Planned: 8 and Workers Launched: 2.

It seems the performance is not different when I use parallel because the
Workers Launched value is only 2.

How can I increase the Workers Launched value to match the Workers Planned
value, or at least raise it to 4?

I want to use parallel to speed up for index creation and data insertion