查一下词典就很清楚了——tissue(纸巾)可以用作可数或不可数名词。
比如《柯林斯高级英语词典》就有如下解释和例句:
[C or U] soft paper which is used for cleaning, especially your nose, and is thrown away after use, or a small rectangular piece of this (尤指擦鼻子用的)纸巾,手巾纸,面纸
She handed me a tissue just as I sneezed. 我打喷嚏时,她递给我一张面纸。
I always keep a box of tissues in the car. 我总是在车里放一盒面纸。
He used a piece of tissue to clean his sunglasses. 他用一张面纸擦太阳镜。