<?php

add_action( 'pre_get_posts', 'raits_change_posts_per_page' );

function raits_change_posts_per_page( $query ) {
    // Change is_home() to whatever you want
    // Just make it select just the stuff you want to affect
    if ( $query->is_main_query() && $query->is_home() )
        $query->set( 'posts_per_page', 1 );
}