tissue是可数名词还是不可数名词,我看到书上有说a tissue,难道是可数名词?

这个词既是可数名词,也是不可数名词
薄棉纸是不可数
科林斯词典：
N-UNCOUNTTissue paper is thin paper that is used for wrapping things that are easily damaged,such as objects made of glass or china.(包装易碎物品的) 薄棉纸
例：
...a small package wrapped in tissue paper.
…一个小包裹,用薄棉纸包着.
作为面巾纸是可数
科林斯：N-COUNT
A tissue is a piece of thin soft paper that you use to blow your nose.面巾纸...a box of tissues.
…一盒面巾纸.
两张面巾纸可以用two tissues,你用two pieces of tissues也对啦