wiki states the pottery arch left and right use 20 pottery bricks and 20 mortar.
I tried to build it and it said;
[04:53:35] You need a fence bars.
So i made one, and sure enough you need one. it seems that the Devs decided that an arch with no support at one end would need fence bars to reinforce it.
This is not the case for a single arch wall, just for the multi-tile spanning arches.
The page for the Arch T is correct however.