Although each carrier may its own regulations regarding MEL, as far as laws are concerned, it seems that we air passengers are entitled to squat (sorry!) when it comes to lavatories on commercial flights. Out of curiosity, I was looking into the topic and the only relevant US law seems to be the Air Carrier Act of 1986, which requires a working lavatory on planes with more than one aisle, that were manufacturered or refurbished after 1992. And there are no requirements that say you need a lavatory per given number of passengers. Apparently, the mighty Airbus A380 could take off on a flight (at least within the US) with 500+ passengers and just one working lavatory and be perfectly within the law.

The law has huge gaps. There are plenty of single aisle planes, like the workhorse Boeing 737. Apparently, they aren't legally required to have any lavatories! I'm not sure if there are any commercial planes that haven't been "refurbished" since 1992, but those would also not be covered under the law. Please tell me we're entitled to more than this under the law!