DropBox is Free but Limited to 3 Devices

As from March 2019 Dropbox is limited to 3 Devices.

The good news is that the change only applies to any new devices you attempt to link to your account. "If you're a Basic user and you linked more than three devices prior to March 2019, all of your previously linked devices will remain linked, but you can't link additional devices," Dropbox said in the help document.

This probably explains why free users haven't noticed the policy update unless they've purchased new hardware.