how does PostgreSQL determine how many parallel processes to start

Hi all,
I know that parallel processes can be limited by
max_parallel_workers_per_gather and max_parallel_workers, as well as
the condition to consider a parallel plan is min_table_scan_size (and
index). But I would like to understand, once a table has been
considered for a parallel plan, and there is room for other workers,
how will PostgreSQL decide to start another process?

Thanks,
Luca