A few months ago, Dropbox introduced a device limit for their free accounts. Users may only link up to three devices with their Dropbox account at any time. At first glance one might think that Notebooks users are now limited syncing their documents to three devices, but fortunately, this is not the case.
The device limit is restricted to the Dropbox app. Third party apps which implement their own sync interface do not count, and Notebooks on iOS is one of these other apps.
So that means you can use and sync Notebooks on up to three Macs or PCs (they need Dropbox’ app for automatic sync), but you can use Notebooks on as many iPads, iPhones and iPods as you want.