How many parallel workers can I have on Postgres?
The limit of concurrent parallel workers for the whole cluster is max_parallel_workers, which must be ≤ max_worker_processes. The limit of parallel workers per query is max_parallel_workers_per_gather.
Adjusting these parameters, you can start as many workers as you please. Just keep in mind that performance will degrade once you are using more parallel workers than your CPU and I/O can handle. Also, don't forget that there may be other queries running, so don't over-allocate resources.
Don't forget alternative approaches: perhaps the query can be rewritten or indexed to be faster.