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add_action( 'pre_get_posts', function( $query ) {
    // Check that it is the query we want to change: front-end search query
    if( $query->is_main_query() && ! is_admin() && $query->is_search() ) {

        // Change the query parameters
        $query->set( 'posts_per_page', 10 );

    }
} );