Cake PHP run multiple Console/cake shell commands in different processes

Question

I've written a Console/cake shell for my Cake PHP 2 project. My shell is called a Queue Manager, and it contains two tasks, a ProcessTask.php and a ScheduleRunTask.php, it's my ProcessTask that once ran will grab entries from a database table, loop over them and do something. I cannot use Supervisor for the project I'm working on as this is a strict prerequisite.

The issue I face is that I need it to run automatically, and ideally need it to automatically scale up and down based on a settings table setting called something like "max workers" so that it's not always running 10 commands at once when it doesn't need to.

I've put together a crontab which runs my scheduler once a minute:

* * * * * cd /path-to-project-app && Console/cake queue_manager schedule_run >> /dev/null 2>&1

When this runs the schedule_run task, this in turn fires up the process task and runs these:

<?php

class ScheduleRunTask extends AppShell {

    /**
    * Main execute task
    */
    public function execute() {
        $maxWorkers = 5;
        $currentWorker = 0

        while ($currentWorker < $maxWorkers) {
            exec("cd /path-ro-my-project-app && Console/cake queue_manager process", $output, $retval);
            $this->out("<info>Worker is running.</info>");
        }    
    }
}

But is this the best way to do this and do these run in their own processes? Which is important.

How do I prevent it from starting up another 5 exec commands on the next minute the cron runs, I'd end up with hundreds right?