The question arises because John’s previous use of the word (Revelation 21:12, Revelation 21:14) was the singular “wall” to describe the (single) structure surrounding the holy city. The Greek in the present verse also gives us the word “wall” in the singular. Presumably, the ESV provides here the plural word “walls” (as do some other translations) because the wall wraps around its four sides, and all sides are measured.
15 And the one who spoke with me had a measuring rod of gold to measure the city and its gates and walls.