Summary Count Of List Items

I have a sheet with a top to bottom date/chronological list of Horse races arranged so;

Column 1 Column 2 Column3 Column4
Date/Time Race type Empty Horse Name

e.g

01/03/2008 12:30:00 1m stks Mad Robber
01/03/2008 12:30:00 1m stks Don't Run
01/03/2008 12:30:00 1m stks Laughing boy
etc

Then next race

i'm looking for a Macro that will add up the number of horses in each race and enter the number for a given race in Column 3 for each horse in that race. So the example above would become;

01/03/2008 12:30:00 1m stks 3 Mad Robber
01/03/2008 12:30:00 1m stks 3 Don't Run
01/03/2008 12:30:00 1m stks 3 Laughing boy

Then next race down the list;

01/03/2008 12:40:00 2m gtd 1 Walk fast

etc