[R] "autonumber" for grouping variable

[clion]

Dear R users,
my dataframe looks like this
head(dat)
   Id  sex byear age
1 300   m  2003  50
2 300   m  2003  36
3 402    f  2003  29
4 402    f  2003  21
5 402    f  2003  64
6 150   m  2005  43
...
...(where Id is just the Identification number of Individual, sex (male or
female), byear (=birthyear))

now, I 'd like to add a column, where each Individual gets an automated
number starting from 1, so that I can number them consecutively
something which should look like this:
   Id sex byear age Number
1 300   m  2003  50 1
2 300   m  2003  36 1
3 402    f  2003  29 2
4 402    f  2003  21 2
5 402    f  2003  64 2
6 150   m  2005  43 3
...
...
how can I easily do this?
thanks a lot for any help,
Birte

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