Number of parallel workers chosen by the optimizer for parallel append

I have a partitioned table, each partition has "parallel_workers = 10" set.

SET max_parallel_workers_per_gather = 8;

SET enable_partitionwise_aggregate = on;

EXPLAIN (COSTS OFF)
SELECT applicant_name, count(ipc_4)
FROM laurenz.z_flat
GROUP BY applicant_name;

QUERY PLAN
--------------------------------------------------
Gather
Workers Planned: 4
-> Parallel Append
-> HashAggregate
Group Key: z_flat_3.applicant_name
-> Seq Scan on xyz_4 z_flat_3
-> HashAggregate
Group Key: z_flat.applicant_name
-> Seq Scan on xyz_1 z_flat
[8 more such partition scans]
(33 rows)

How does the optimizer decide to use 4 parallel workers?

No matter what I try, I cannot influence that number.

Yours,
Laurenz Albe