I am guessing a running total would work, but that is not my forte. Shared variables would work as well.

typically in groups of 3 formulas for shared variables
1) reset, usually a group header:
shared numbervar x := 0;
"" //will hide the reset

2) increment, usually in the detail section:
this would by your existing formula with a minor alteration:
shared numbervar x;
if {my.table}=123 then(x:=x+1; "Yes") else if {my.table}=321 then "No"

3) display, usually in a group footer:
shared numbervar x

HTH