//search.php
echo '<a href="'. add_query_arg( 'count', '10' ) .'" title="View 10 posts per page">10</a>';
echo '<a href="'. add_query_arg( 'count', '50' ) .'" title="View 50 posts per page">50</a>';
echo '<a href="'. add_query_arg( 'count', '100' ) .'" title="View 100 posts per page">100</a>';

//functions.php
function wds_btc_limit_search_posts() {
	if ( is_search() ) {
		switch($_GET['count']) {
			case '50':
			$_SESSION['search_count'] = $_GET['count'];
			break;
			case '100':
			$_SESSION['search_count'] = $_GET['count'];
			break;
			default:
			$_SESSION['search_count'] = '10';
		}
		$limit = $_SESSION['search_count'];
	}
	set_query_var( 'posts_per_page', $limit );
}
add_filter('pre_get_posts', 'wds_btc_limit_search_posts');