Of course, we could make this value 1-based rather than 0-based, as Peter Geoghegan suggested a while back. But as I think I said at the time, I think that's more misleading than helpful. The leader participates in the parallel plan, but typically does far less of the work beneath the Gather node than the other nodes involved in the query, often almost none. In short, the leader is special. Pretending that it's just another process involved in the parallel group isn't doing anyone a favor.
Does this apply to the extent that a value of 1 is likely worse than 0 since the leader is now tasked with accumulating but there is only one process actually working to provide the leader data?
I'm inclined to accept max_parallel_workers where a value of 0 means no parallelism and the non-zero counts indicate the number of workers in addition to the required leader.
Though that does suggest "additional" as a valid option. Something like "max_additional_workers". Just how overloaded is the term "worker". If worker is understood to mean "a process which implements execution of [part of] a query plan" the word additional leaves no ambiguity as to where the leader is accounted for.
It does significantly reduce grep-ability though :(