But Woolie had grown so much fleece that he didn't even look like a sheep.

可以说，所有的词典都把 fleece 看作可数名词。两层意思：指 “羊毛”；指“从一只羊身上剪下连在一起的羊毛”。用了复数时，可以指许多根羊毛，也可以指许多只成片的羊毛。

原句作者误认为 fleece 和 wool 都不可数，所以，用了so much fleece，应该是 so much wool 或so many fleeces。fleeces 用复数举例：

These sheep have fine thick fleeces. 

这些绵羊的毛又细又厚。