When a project isn't actively being worked on, you can archive it so that it no longer counts against your plan's project limit. Once archived, a project can still be viewed, it just can't be edited or commented on until it's made active again.
To archive a project, go to your project's thumbnail page and click on 'Project settings'.
Once you're on the project's settings page, click the 'Permissions & other settings' tab. There you'll see the field 'Project status' which lets you mark a project as either 'active' or 'archived'.
Once you've archived a project you'll see a new 'Archived projects' link in your dashboard sidebar.
Clicking that link takes you to a list of all your archived projects, where you can browse through the old mockups and comments and also change the project's status back to active.