Is there a way to change permission to more than one item at once? If I add 100 items do I have to go to each one open it and change the permissions?
By “item” do you mean media file or layout?
Layout permissions can be assigned with Cascade permissions... which will set the same permissions to all regions and widgets on that layout.
If you meant media files, then yes that would need to be done one by one, alternatively you can set the default permissions for newly uploaded files in the CMS Settings page.
You can set that to
private (default)
group (view)
group write (view/edit)
public (view)
public write (view/edit