You would have that number of MATLAB processes running and waiting for work, consuming computing resources from your machine. You can see this from whatever task list application exists on your platform.
Assuming your machine has the capacity to run those processes plus whatever you are doing serially there's not a downside. It would be the same as if you have some other number of arbitrary programs open while you are using MATLAB -- as long as everything stays in RAM and you have adequate processors to support it, you'll likely not notice.
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