Can't increase the number of recent documents in gedit

This option has been removed in recent versions of gedit, so the dconf key was also removed.

A workaround is to use the Quick Open plugin, which shows more than 10 recent files. As a bonus, you can also search for files with it.

Install, if you haven't already the gedit-plugins package, which contains the Quick Open plugin along with a bunch of other useful plugins:

sudo apt install gedit-plugins

To enable the plugin click the hamburger menu ≡ → Preferences → Plugins and enable Quick Open (see screenshot below).

After that press Ctrl+Alt+O to open a window which shows your recently opened files and also allows to search for files to open.