It's a small shop, with an average of about 500 customers a week.

【翻译】这是一家小店，平均每周有500名顾客光顾。

句子主干为主系表结构。

重点分析with 短语：

最佳分析办法是，把 an average of 看作限定语，修饰about 500 customers，而about 500 customers 是with 的介宾。介词with 表示“伴有”之意，类似 He is a boy with blue eyes. 中with 的用法，整个with an average of about 500 customers a week 作定语，修饰shop。

a week = every week 表示“频度”。