Edit your /etc/sysctl.conf file with your favourite text editor, e.g. nano:

sudo nano /etc/sysctl.conf

and append (or modify the value of) fs.inotify.max_user_watches=100000 at the end of the file as a new line.

As an option you can use the command sudo sysctl -w fs.inotify.max_user_watches=100000 or the command below, as described on the Dropbox help guide, the results will be the same.

From the advanced help guide from Dropbox

Monitoring more than 10000 folders

The Linux version of the Dropbox desktop application is limited from monitoring more than 10000 folders by default. Anything over that is not watched and, therefore, ignored when syncing. There's an easy fix for this. Open a terminal and enter the following:

echo fs.inotify.max_user_watches=100000 | sudo tee -a /etc/sysctl.conf; sudo sysctl -p

This command will tell your system to watch up to 100000 folders. Once the command is entered and you enter your password, Dropbox will immediately resume syncing.