This feature has not yet been released, it's only on puma/main

Have to wait for Puma 6.4.3, I guess!

Author: dhh

railties/lib/rails/generators/rails/app/templates/config/puma.rb.tt

@@ -25,7 +25,7 @@
 # be configured to provide at least as many connections as the number of
 # threads. This includes Active Record's `pool` parameter in `database.yml`.
 threads_count = ENV.fetch("RAILS_MAX_THREADS", 3)
-threads threads_count
+threads threads_count, threads_count
 
 # Specifies the `environment` that Puma will run in.
 rails_env = ENV.fetch("RAILS_ENV", "development")