Yes. The MEL says how many lavs you need to dispatch based on the length of flight.

You probably could go will all of them out if it was oh, say, a 30 minute flight.
I don't have a MEL in front of me so I cannot say for sure. I do know it is up to the captain to go with the 1st class (front) lav inop due to crew security issues. What was the story?