Query with number of occurances.

Hi there, had a quick question about finding the number of occurrences of specific column. Let me give a quick example.

Table t:
col_1 | col2 | col_3
A | B
A | C
A | C
A | D
A | D
A | D

I want to put the number of occurrences of col_2 in a col_2.
should output:

col_1 | col2 | col_3
A | B | 1
A | C | 2
A | C | 2
A | D | 3
A | D | 3
A | D | 3

Any ideas what I can do for this?
Thank you very much.

select col_1, COUNT(col_1) as col2 into #temp from tablet group by col_1
update Tablet set col2 = a.col2 from tablet join #temp a on tablet.col1 = a.col1

Wow thanks a lot. I know it's simple but I was using the count wrong.
Much appreciated :)

Happy SQLing!