区别就是两者单复数形式是不一样，具体的不同如下
quantity of为单数形式，中文意思是大量的;
It's just very time consuming to get such a large quantity of data
获取如此大量的数据非常费时
quantities of为复数形式中文意思是
大量的;
例句
＂ Not only are the quantities of phytoplankton in the ocean changing, ＂ she said, ＂ but the type of phytoplankton is changing ．＂
"不仅海洋中浮游植物的数量在变化,"她说,"而且浮游植物的种类也在变化。"
