You’re passing two parameters to _n function, the first one is the pattern for one item, and the second for 2 or more. Just change the pattern.
Show only numbers:
<?php echo sprintf (_n( '%d', '%d',count( WC()->cart->get_cart())),count( WC()->cart->get_cart())); ?>
Show item for two or more:
<?php echo sprintf (_n( '%d', '%d items',count( WC()->cart->get_cart())),count( WC()->cart->get_cart())); ?>
For more information check here.
But, if you want to show only the number, you could echo the cart count directly:
echo count( WC()->cart->get_cart());
or
echo WC()->cart->get_cart_contents_count();