you'll need the package lubridate to calculate which week is the given date at first. then you may use group_by and summarise function to summarise data according to week_of_year.
for example:
library(lubridate)
df1 <- data.frame(
id_customer = sample(1:30,100,replace = T),
date = sample(as_date('2019-1-1'):as_date('2019-6-30'),100,replace = T) %>% as_date
)
df1 %>% mutate(week_of_year = week(date)) %>% group_by(week_of_year) %>%
summarise(number_of_customers = sum(id_customer))