您可以用 "the number of people being served" 来表示 "服务的人数"。这个短语的来源是由 "number" 和 "people being served" 两个词组合而来。
例句：
The number of people being served at the restaurant varies depending on the time of day.（餐馆服务的人数因时间而异。）
The number of people being served by the charity organization has increased significantly in the past year.（慈善机构服务的人数在过去一年中显著增加。）
The number of people being served by the health cpnic is pmited by the availabipty of medical staff.（医疗诊所服务的人数受医务人员数量的限制。）