cli.rb

module ReportCli
  class Runner

    include Concurrent::Async
    
    attr_accessor :threads_num, :run_times, :caller

    def initialize
      @conn = Config.connection
    end

    # TODO use TPoolexecutor to limit queue ,
    # http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/ThreadPoolExecutor.html

    def run
      pool = Concurrent::FixedThreadPool.new(self.threads_num)
      
      @instance = self
      
      pool.post{ 
        @instance.run_times.times{ 
          @instance.caller.exec 
        } 

      }
      pool.wait_for_termination
      
    end

    def call
      puts "sending reports"
      self.caller.send_report
    end

  end

end

example.rb

      runner  = PreyReportCli::Runner.new
      runner.threads_num = 5
      runner.run_times = 5
      runner.caller = PreyReportCli::Report.new
      runner.run

report.rb

module ReportCli
  class Report
    
    def initialize
      @conn = Config.connection
    end

    def end_point
      "/api/v2/devices/#{Config.device_key}/reports.json"
    end

    def exec
      payload = {}
      payload[:screenshot] = Faraday::UploadIO.new("./#{shuffled_images}.jpg", 'image/jpeg')
      @conn.post end_point, payload
    end

    def shuffled_images
      ["mat","pic","van"].shuffle.first
    end

  end

end

Also, your Report class may not be thread safe. I don't know what you are storing in Config.connection but that needs to be thread safe for this to work. If it isn't thread safe, your code isn't thread safe. I'm going to assume that it is thread safe.

@michelson Please take a look at this: https://gist.github.com/jdantonio/baf400950605e7e01f36