a large number of:He has a large number of friends (他有很多朋友).
a large quantity of:A large quantity of beer was sold (售出了大量的啤酒).
a mount of:This library houses a mount of learing works (这座图书馆有许多学术著作).
从以上例句们看出,通常a large quantity of修饰不可数名词,另两个修饰可数名词的.