Here is an example of adding two default layouts to a new post
add_filter('acf/load_value/name=flexible', 'add_starting_repeater', 10, 3);
function add_starting_repeater($value, $post_id, $field) {
if ($value !== NULL) {
// $value will only be NULL on a new post
return $value;
}
// add default layouts
$value = array(
array(
'acf_fc_layout' => 'layout_1' // layout_1 is the name of the layout
),
array(
'acf_fc_layout' => 'layout_1'
)
);
return $value;
}