@meneldil My quick go to your question as I was searching for the answer myself.
if( have_rows('my_repeater') ):
$i = 0;
while( have_rows('my_repeater') ): the_row();
if( get_sub_field('sub_field_boolean') ) $i++;
endwhile;
$tCount = $i;
endif;
($tCount to produce the number of rows with a true field)