How to manage number of Sandboxes allowed

Summary

I'm getting an error saying there are too many Sandboxes, need to expand the number allowed

Content

Hi all,

       In one of our Dev environments that was just updated to 20D, I was doing some Page Composer changes to match our Production setup, but today I am getting an error message saying there are too many Sandboxes open and I have to close some or change the number of Sandboxes allowed.  There are only 3 open sandboxes, and maybe 15 published ones, much less than some of our Dev environments, so I'm not sure what is causing this issue.  Does anyone know how to change the setting of how many Sandboxes are allowed, or some other method