I was recently working on one of my badges on TrailHead and when I tried to create a new playground, I ended up getting the error: ''You have reached the maximum of 10 Trailhead playgrounds"! Here are the steps on how to resolve that:
Click on the avatar/your profile and then click on 'Hands-On orgs' (as shown in the image above)
You will see the list of all the TrailHead playgrounds you created, click on the cross sign (see image) for the playground that you don't need anymore and then select 'disconnect'
You would now be allowed to create new playground for your modules.