If I got your question right, AFAIK there is no direct way to count the actually displayed rows.
You either need to do some calculations using arrayctrl's display position and rows' rectangle OR if your array has some child ctrls, then you might want to check their visibility state (I'm not 100% sure but this might work) and then count them.
OTOH, if you just want to retrieve the total number of rows (both visible and invisible), you can use