Using the grep command to filter

Hello Everyone,

I am a college student currently enrolled in a Unix class. Unix is in
no way shape or form related to my major, and needless to say, I'm
stumped.

The question I must answer is this:

"Enter a single command-line that would find out the 5 files in/under
/etc that contain the string 'ubuntu,' case insensitively, more than
any other file in/under the /etc directory. Sort the list from most to
least and throw away standard error."

I can do each part the question asks separately, but I've no idea how
to put this all together to create a single working command. Any help
would be greatly appreciated.

Thanks,
D.

.