id      date        count
4668    2017-01-01  1
4771    2017-01-01  8
4883    2017-01-01  2
4884    2017-01-01  6

> data_filled <- data%>%
+     mutate(Date = as.Date(date)) %>%
+     complete(Date = seq.Date(min(date), max(date), by="day"),id)

>data_filled <- data%>%
   +  complete(nesting(id, count), date = seq(min(date), max(date), by = "day"))