If I create a simple rule like

rule '.o' => ['.c'] do |t|
  sh "cc #{t.source} -c -o #{t.name}"
end

How can I tell Rake that I want the auto-generated tasks to be parallelizable?

as stated by Dennis Rake rules are implemented as Tasks

therefore the -m flag which converts tasks to multitasks also converts rules to 'multirules'

require 'rake/clean'

rule '.ext2' => '.ext1' do |t|
    sh "cp #{t.source} #{t.name}"
    sleep(1)
end

def dependencies(input_file)
    base, is, ext = input_file.split('.')
    _from, _to = is.split('-')
    files = []
    Integer(_from).upto(Integer(_to)) do |i|
        files << "#{base}_#{i}.#{ext}"
    end
    return files
end

rule ".ext2" => lambda { |i| dependencies(i) } do |t|
    sh "touch #{t.source}"
end

task :make_files do |t|
    1.upto(20) do |i|
        sh "touch file_#{i}.ext1"
    end
end

CLEAN = FileList['*.ext2']

running the following commands (not threaded):

rake make_files
time rake file.1-20.ext2

real    0m20.232s
user    0m0.134s
sys     0m0.082s

with 5 threads:

rake clean
time rake -m -j 5 file.1-20.ext2

real    0m4.152s
user    0m0.122s
sys     0m0.071s

with 20 threads:

rake clean
time rake -m -j 20 file.1-20.ext2

real    0m1.167s
user    0m0.130s
sys     0m0.065s

I use sleep(1) to emulate 'work/io', if your processes block a lot or if you have many cores this might be useful to you :)

EDIT: As pointed out by Shadow in comments, the follow toggles 'multitask' to be always on

Rake.application.options.always_multitask = true