test.rb

require 'rspec'
 
class Bar
  def self.bar(num)
    puts "#{num}: Called!"
  end
end
 
class Foo
  def self.foo(num = 100)
    threads = []
    (1..num).each do |i|
      threads << Thread.new do
        Bar.bar(i)
      end
    end
    threads.map(&:join)
  end
end
 
describe Foo do
  context 'many' do
    100.times do
      it 'works' do
        num = 2
        #allow(Thread).to receive(:new).and_yield.and_return(Thread.new {})
        expect(Bar).to receive(:bar).exactly(num).times.and_call_original
        Foo.foo(num)
      end
    end
  end
end