Franco, there's no built in method to mass-archive like Jer said, but the fastest method right now would still be marking them read only from the Portal, but that requires your projects to be cloud data. You still can't mass-archive them but it's far fewer clicks to archive them than in the Project List in CAD. However, your office is Local Data, so the Project List is your only method.
On the other hand, if you want to take this opportunity to switch from Local Data to cloud data you can (for free, it's included already for everybody) and get all the additional benefits like full 3D plugin access, very reliable backups through us and you still get local backups, more remote work possibilities, and easier technical support from us then now would be a good time. Part of the process is backing up all projects and preference sets and then mass-uploading them to cloud data. It takes under 30min and you could just not upload all those projects you want to archive while still having that backup of them that you just made to restore later if needed. That would be like mass-archiving and as much time if not less than individually archiving all the projects.
Here's more information on cloud data, switching steps, and the portal.
Cloud Data Overview
Cloud Data vs. Local Data (Database Installation)
Switch to Cloud: Video at the top
How to Use the Land F/X Portal