Thank you John,
I solved it using the loop have rows:
$number_of_banners = 0;
if( have_rows('collections') ):
while ( have_rows('collections') ) : the_row();
if( have_rows('banners') ):
while ( have_rows('banners') ) : the_row();
$number_of_banners +=1;
endwhile;
endif;
endwhile;
endif;
<p><?= $number_of_banners === 1 ? $number_of_banners . " banner" : $number_of_banners . " banners" ?></p>
I tried with the correct loop using array and it return 5 instead of 8 which is the number of only 1 banner repeater.
This is solved, however I would like to improve it using the loop array.