I dont know a way to do this. The logic of the report as I understand it is this. The main report records are selected then the subreport records are selected. The subreport is generally linked to the main report by a parameter or field. So if that is true then logically the count you want does not exist at the time the data for the main report is selected.
There a couple of way to get your count into the main report data query using SQL. The most common would be through a join and/or a subSelect statement.
You may also be able to create and SQL Expression as part of the Selection formula.
If you post a sample of your data queries for the main report and subreport someone will likely be able to help with more specific suggestions.
Regards,
John W.