custom function for gallery image count

child-theme.php

<?php

function acf_gallery_count($field_name, $post_id = null) {
    $gallery = get_field($field_name, $post_id);
    $count = is_array($gallery) ? count($gallery) : 0;
    return $count;
}

remember to add the function name to the list of allowed custom functions - https://academy.bricksbuilder.io/article/filter-bricks-code-echo_function_names/

add_filter( 'bricks/code/echo_function_names', function() { return [ 'acf_gallery_count', ]; } );

Then use within the echo function, replacing 'gallery' with your field name

{echo:acf_gallery_count('gallery')}