I have not modified advancedsettings.xml myself, so this is just my read based on the the link you provided.
The first thing I noticed is the disclaimer that “The advancedsettings.xml file does not exist by default. You have to create it first!”. So if you have not already created it yourself (in which case you would know where it is), I can understand why you might have trouble finding it! 
The site also says that if you want to create “advancedsettings.xml”, it should go in the userdata folder. The userdata folder resides inside the kodi folder. And the kodi folder is hidden, so this would also have made things tricky to find.
When using ssh, if you specify -a in your list command, it will show all, and then you will see the .kodi folder I mentioned:
It sounds like in that userdata folder, you will need to create a file called “advancedsettings.xml”, and populate the file with:
<advancedsettings>
<videolibrary>
<recentlyaddeditems>50</recentlyaddeditems>
</videolibrary>
</advancedsettings>
You can do that through ssh directly. Or, if you are not so comfortable with ssh, you can also create advancedsettings.xml separately (in a text editor on Windows, for example), and then use a GUI-type ftp client (like FileZilla or WinSCP) to upload to the location described above.
