diff --git a/activesupport/lib/active_support/test_case.rb b/activesupport/lib/active_support/test_case.rb
index ef12c6b9b0..7be4108ed7 100644
--- a/activesupport/lib/active_support/test_case.rb
+++ b/activesupport/lib/active_support/test_case.rb
@@ -12,6 +12,7 @@ require "active_support/testing/constant_lookup"
 require "active_support/testing/time_helpers"
 require "active_support/testing/file_fixtures"
 require "active_support/testing/parallelization"
+require "concurrent/utility/processor_counter"
 
 module ActiveSupport
   class TestCase < ::Minitest::Test
@@ -58,16 +59,20 @@ module ActiveSupport
       # If the number of workers is set to +1+ or fewer, the tests will not be
       # parallelized.
       #
+      # If +workers+ is set to +:number_of_processors+, the number of workers will be
+      # set to the actual core count on the machine you are on.
+      #
       # The default parallelization method is to fork processes. If you'd like to
       # use threads instead you can pass <tt>with: :threads</tt> to the +parallelize+
       # method. Note the threaded parallelization does not create multiple
       # database and will not work with system tests at this time.
       #
-      #   parallelize(workers: 2, with: :threads)
+      #   parallelize(workers: :number_of_processors, with: :threads)
       #
       # The threaded parallelization uses minitest's parallel executor directly.
       # The processes parallelization uses a Ruby DRb server.
-      def parallelize(workers: 2, with: :processes)
+      def parallelize(workers: :number_of_processors, with: :processes)
+        workers = Concurrent.physical_processor_count if workers == :number_of_processors
         workers = ENV["PARALLEL_WORKERS"].to_i if ENV["PARALLEL_WORKERS"]
 
         return if workers <= 1
diff --git a/guides/source/testing.md b/guides/source/testing.md
index 9541598b26..f34f9d95f4 100644
--- a/guides/source/testing.md
+++ b/guides/source/testing.md
@@ -473,8 +473,8 @@ takes your entire test suite to run.
 ### Parallel testing with processes
 
 The default parallelization method is to fork processes using Ruby's DRb system. The processes
-are forked based on the number of workers provided. The default is 2, but can be changed by the
-number passed to the parallelize method.
+are forked based on the number of workers provided. The default number is the actual core count
+on the machine you are on, but can be changed by the number passed to the parallelize method.
 
 To enable parallelization add the following to your `test_helper.rb`:
 
@@ -516,7 +516,7 @@ class ActiveSupport::TestCase
     # cleanup databases
   end
 
-  parallelize(workers: 2)
+  parallelize(workers: :number_of_processors)
 end
 ```
 
@@ -531,7 +531,7 @@ To change the parallelization method to use threads over forks put the following
 
 ```ruby
 class ActiveSupport::TestCase
-  parallelize(workers: 2, with: :threads)
+  parallelize(workers: :number_of_processors, with: :threads)
 end
 ```
 
diff --git a/railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt b/railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt
index c06cd525d7..47b4cf745c 100644
--- a/railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt
+++ b/railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt
@@ -5,9 +5,9 @@ require 'rails/test_help'
 class ActiveSupport::TestCase
   # Run tests in parallel with specified workers
 <% if defined?(JRUBY_VERSION) || Gem.win_platform? -%>
-  parallelize(workers: 2, with: :threads)
+  parallelize(workers: :number_of_processors, with: :threads)
 <%- else -%>
-  parallelize(workers: 2)
+  parallelize(workers: :number_of_processors)
 <% end -%>
 
 <% unless options[:skip_active_record] -%>
diff --git a/railties/test/application/test_runner_test.rb b/railties/test/application/test_runner_test.rb
index 140703e118..55cfda82ea 100644
--- a/railties/test/application/test_runner_test.rb
+++ b/railties/test/application/test_runner_test.rb
@@ -523,6 +523,8 @@ module ApplicationTests
     end
 
     def test_run_in_parallel_with_processes
+      substitute_arguments_of_parallelize_method("workers: 2, with: :processes")
+
       file_name = create_parallel_processes_test_file
 
       app_file "db/schema.rb", <<-RUBY
@@ -540,11 +542,7 @@ module ApplicationTests
     end
 
     def test_run_in_parallel_with_threads
-      app_path("/test/test_helper.rb") do |file_name|
-        file = File.read(file_name)
-        file.sub!(/parallelize\(([^\)]*)\)/, "parallelize(\\1, with: :threads)")
-        File.write(file_name, file)
-      end
+      substitute_arguments_of_parallelize_method("workers: 2, with: :threads")
 
       file_name = create_parallel_threads_test_file
 
@@ -563,6 +561,8 @@ module ApplicationTests
     end
 
     def test_run_in_parallel_with_unmarshable_exception
+      substitute_arguments_of_parallelize_method("workers: 2, with: :processes")
+
       file = app_file "test/fail_test.rb", <<-RUBY
         require "test_helper"
         class FailTest < ActiveSupport::TestCase
@@ -587,6 +587,7 @@ module ApplicationTests
     end
 
     def test_run_in_parallel_with_unknown_object
+      substitute_arguments_of_parallelize_method("workers: 2, with: :processes")
       create_scaffold
       app_file "config/environments/test.rb", <<-RUBY
         Rails.application.configure do
@@ -966,6 +967,14 @@ module ApplicationTests
         RUBY
       end
 
+      def substitute_arguments_of_parallelize_method(arguments)
+        app_path("test/test_helper.rb") do |file_name|
+          file = File.read(file_name)
+          file.sub!(/parallelize\(([^\)]*)\)/, "parallelize(#{arguments})")
+          File.write(file_name, file)
+        end
+      end
+
       def create_env_test
         app_file "test/unit/env_test.rb", <<-RUBY
           require 'test_helper'