How to show more post at a given category?

put this code in functions.php :

function change_numberposts( $query ) {
    // Only modify the main query
    if( $query->is_main_query() ){
        // Home page
        if ( $query->is_home() ) {
            $query->set( 'posts_per_page', 6 );
        }
        // Category 21
        if ( $query->is_category( 21 ) ) { // parameter for is_category may be Category ID, Category Title, Category Slug or Array of IDs, names, and slugs.
            $query->set( 'posts_per_page', 10 );
        }
    }
}
add_action( 'pre_get_posts', 'change_numberposts' );

Links:

is_category