/etc/sysctl.conf file with your favorite text editor, ie
sudo nano /etc/sysctl.conf
fs.inotify.max_user_watches=100000 in the end of the file as a new line.
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.