Total number of stereoisomers of truxillic acid
At first glance, not only does it seem like there are no stereogenic centres in truxillic acid, it is also highly symmetrical. However, if we consider different conformations of the chemically identical units (H, COOH or Ph), which give rise to different isomers, some carbons at the 4-membered ring can possibly become stereogenic.

Stereogenic centres are labelled with an asterisk: ε-truxillic acid and peri-truxillic acid do not have stereogenic carbon centres as their 4-membered ring carbons have chemically and conformationally identical neighbours.
It took me a while to get this.Thanks to @andselisk and wikipedia.