请问:wall 什么时候用单数，什么时候用复数？

意为壁炉时，有复数
意为墙时，没有复数

我查了字典，wall只当可数名词，表示墙，似墙之物，街道的靠墙部分。下面是朗文对wall（n）这个单词的解释
1
▶AROUND AN AREA◀
an upright flat structure made of stone or brick, that divides one area from another or surrounds an area
ￚsee also fence
stone/brick/concrete wall The estate is surrounded by high stone walls. city/garden etc wall the ancient city wallsthe Great Wall of ChinaWe climbed over the wall into the orchard.
2
▶IN A BUILDING◀
one of the sides of a room or building
on the wall
I put some pictures up on the walls .
Bob leaned against the wall . bedroom/kitchen etc wall We decided to paint the bathroom walls blue.
3
▶BODY◀
the side of something hollow, especially within the body
The walls of the blood vessels had been damaged. cell walls
4
wall of fire/water etc
a tall mass of something such as fire or water, that stops anything from getting past
The boa