create a formula in your subreports that use a shared variable and increment it in each subreport...depending on what you want...
shared numbervar rpt1:= some CR expression
you can either update this one shared variable for all of you reports, reporting back the total or each subreport can have its own variable, then to display/manipulate them in the main report just create a formula that access the same variable:
shared numbervar rpt1 //this will display the result from subreport 1 in the main report.
HTH