df <- data.frame(Customer_ID=c(1,1,1,2,2,2,3,3,4,4),
                 Transaction_ID=c(101,102,103,104,105,106,107,108,109,110),
                 Date=as.Date(c("2019-01-01","2019-01-08","2019-01-15","2019-01-02","2019-01-20","2019-01-21",
                                "2019-01-02","2019-01-06","2019-01-03","2019-01-19"),format="%Y-%m-%d"),
                 Order_Number=c(1,2,3,1,2,3,1,2,1,2))

answer <- data.frame(Date=as.Date(c("2019-01-01","2019-01-02","2019-01-03","2019-01-04","2019-01-05","2019-01-06","2019-01-07","2019-01-08")),
                               customers=c(2,4,4,4,4,3,3,2))