how to number in vba

Basically what code would I use to add numbers to data, for example. if I add 10 rows of data in column A. how would I number it in column b 1 to 10.
But what I also really what with it is for the code to be able to deal with different numbers of rows. for example, on one day I might have 5 rows of data so I want it to number 1 to 5. but the next day I might have 20 rows of data so I want it to number 1 to 20. cheers guys.

this is what i want to do

a1 b1

a 1
aa 2
a 3
a 4
a 5
a 6