acf_set_rows.php

<?php

function set_rows( $selector, $value = array(), $post_id = false ) {

	// filter post_id
	$post_id = acf_get_valid_post_id( $post_id );

	// get field
	$field = acf_maybe_get_field( $selector, $post_id, false );

	// bail early if no field
	if ( ! $field ) {
		return false;
	}

	// update value
	acf_update_value( $value, $post_id, $field );

	// return
	return count( $value );

}