Count the number of observations in the data frame in R

I want to know the way of counting the number of observations using R.
For example, let’s say I have a data df as follows:

df <- data.frame(id = c(1,1,1,2,2,2,2,3,3,5,5,5,9,9))
Even though the biggest number of id is 9, there are only 5 numbers: 1,2,3,5,and 9. So there are only 5 numbers in id. I want to count how many numbers exist in id like this.

>Solution :

In base R:

length(unique(df$id))
[1] 5

Here, unique filters only distinct values and length then counts the number of values in the vector

In dplyr:

df %>%
  summarise(n = length(unique(id)))

Alternatively:

nrow(distinct(df))

Here, distinct subsets the whole dataframe (not just the column id!) to unique rows before nrow counts the number of remaining rows