---- number of people poured into the cinema,and I lost ---

a number of是形容很多的意思,the number of是指数量.比如The number of students in my school is 2000.(后面要用is,因为是指具体的数）.an就不用说了,number不是以元音音素开头的自然不能用an了.lost sight of 是因定搭配,中间不能跟冠词.