the first problem I see is that you are trying to use get_sub_field() outside of a have_rows() loop.
correct loop using have_rows()
if (have_rows('collections')) {
while (have_rows('collections')) {
the_row();
$banners = get_sub_field('banners');
}
}
correct loop using array
$collections = get_field('collections');
foreach ($collections as $collection) {
$banners = $collection['banners'];
}
The other thing is this $banners.count(); should be count($banners)
