Use this code to show ten rows in your repeating section by default. Replace '10' with any number and replace '508' with the ID of your repeating section.

add_filter( 'frm_repeat_start_rows', 'frm_set_ten_rows', 10, 2);
function frm_set_ten_rows( $row_num, $field ){
    if ( $field['id'] == 508 ) {
        $row_num = 10;
    }
    return $row_num;
}