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。