表示废弃物（尤指在公共场所乱扔的废纸）、猫砂、垫草时是不可数名词,
eg.The streets were full of litter.街道上满是乱扔的废纸,
其它情况litter和many是反义词并且都是可数名词.
百度词典给出的解释如下：
1.废弃物,零乱之物[U]
There were piles of litter on the streets.
街上有一堆一堆的废弃物.
2.杂乱[S]
Her room was in such a litter that she was ashamed to ask me in.
她的房间十分杂乱,她不好意思请我进去.
3.(动物睡窝里垫的)干草等物;厩肥[U]
4.(猪、狗等生下的)一窝(仔畜)[C][G]
A female rat may have five or six litters yearly,and an average of nine per litter.
一只雌鼠每年可产幼仔五至六窝,平均每窝九只.
5.(森林里的)枯枝落叶层[U]
6.轿,舆;担架[C]
其中,
[U] 表示不可数名词.通常不与a/an/one连用,无复数,后接单数动词,用little/much等修饰.
[C] 表示可数名词.可以用a/an/one修饰,亦可加-(e)s构成复数,用few/many等修饰.
[S] 表示指该词只用单数形,通常与不定冠词连用.