There is not a built-in way to count items per filter or rule in an “ad hoc” way–at least that I’m aware of…
You could perhaps build in this functionality by adding some conditional cases in your OnItemLoad event. For instance, in your example, your repeater dataset would have a column for Name and a column for Gender. You could have conditional cases with “IF [[Item.Gender]] contains “Boy” set value of CountBoy to [[CountBoy + 1]]” and “IF [[Item.Gender]] contains “Girl” set value of CountGril to [[CountGirl + 1]]” --where CountBoy is a global variable (or text on a hidden widget if you don’t like too many global variables.) Of course, you’d want to initialize these variables to zero before counting your boys and girls, so preface those cases with a conditional case like “IF [[Item.isFirst]] equals “true” set CountBoy to 0”.
I haven’t tried this out, but it should work with filtered repeaters.
I believe this would always evaluate to “false” because the .visibleItemCount property is a numeric value.