Basic users have a three device limit as of March 2019.
Plus, Family, and Professional users can link as many devices as they need.
Business users can link as many devices as they need, but Advanced and Enterprise Dropbox Business admins can limit the number of devices that their teams can link.
What do I do if my device limit is reached?
If you're a Basic user and you linked more than three devices prior to March 2019, all of your previously linked devices remain linked.
However, if you'd like to link a new device (or re-link a previously linked device), you must unlink all currently linked devices (down to less than three), and then you can link your new devices (up to three).
To link more than three devices, upgrade your Dropbox account.