Which is the airliner with the most passengers per lavatory?
This is not necessarily an attempt to answer your question directly. But I do have some interesting information. 

I use to be the night shift supervisor for Continental Airlines in Anchorage Alaska from 1998 to 2005. We primarily serviced B757 and B737 flights from Newark NJ or Houston TX via Portland or Seattle each day (4 winter, 6 summer flights). About half the flights had lavatory problems.

The MEL (minimum equipment list) only required the lavatory closest to the cockpit to be operational for a flight. We averaged about 190 pax (max was 230-290). Imagine a B757 with 200 passengers with only one lavatory for a 5hr flight between NJ and WA (WA to AK was 3hrs) - how inhumane! And I did rarely see flights with only two toilets that worked (never one). 

This was an eye opener to me that the FAA (govt) is not looking after the best interest of the public. I could tell you other bone chilling experiences but this is not the proper place or time :(

So with due respect, I am warning that there is a difference between the number of toilets in an airplane, and the number of toilets that actually work!