I believe in two ply. Why are there toilet paper rolls in public restrooms that are as big as C172 tire and force you lean opposite your normal lean? Because someone decided to “save money” with one ply, not considering that you now have to use twice as much, or more, resulting in little rolls littering the stall (even on makeshift wire hangers, remember those?), or a giant roll that MAY last between janitorial cleanings. Not to mention that once that large roll starts rolling it’s inertia prevents you from grasping the loose end. Plans are currently underway to cross purpose the 737 Max nose wheel snubber to slow the roll down to allow easier grasping of the end.

Useage is mathematically depicted in the following "ply angle" graph, where E-x describes the quantity of two ply use over time and E-y describes the quantity of one ply use over time.