No need to edit your parent themes files as its bad practice.
You can use this code in your child themes functions file
add_action( 'pre_get_posts', 'wpsites_limit_posts' );
function wpsites_limit_posts( $query ) {
if( $query->is_main_query() && !is_admin() && is_home() ) {
$query->set( 'posts_per_page', '1' );
}
}
Won’t work when admin is logged in.
Change the is_home() conditional tag to the archive you want to limit posts per page.