create index insist on 2 workers only

We 're rebuilding a big table which has set parallel_workers = 6

system has

 max_parallel_maintenance_workers | 6       | /var/lib/pgsql/11/data/postgresql.sumup.conf
 max_parallel_workers             | 16      | /var/lib/pgsql/11/data/postgresql.sumup.conf
 max_parallel_workers_per_gather  | 4       | /var/lib/pgsql/11/data/postgresql.sumup.conf

Also session level on index restore there is 

set max_parallel_maintenance_workers = 6;

Still we get only 2 parallel processes in a free of any other load system.

It is postgres 11.7 

Where to look at ?

Thank You very much

Rado