This functions limits the number of items in WooCommerce cart to only one.

allow_only_one_subscription.php

<?php
if ( ! function_exists( 'allow_only_one_subscription' ) ) {

	/**
	 * allow_only_one_subscription.
	 *
	 * This functions limits the number of items in
	 * WooCommerce cart to only one.
	 * 
	 * @since 1.3.2
	 */
	function allow_only_one_subscription( $valid, $product_id, $quantity ) {

	    if( ! empty ( WC()->cart->get_cart() ) && $valid ){
	        WC()->cart->empty_cart();
	        wc_add_notice( 'Whoa hold up! You can buy only one subscription at a time.', 'error' );
	    }

	    return $valid;

	}
	add_filter( 'woocommerce_add_to_cart_validation', 'allow_only_one_subscription', 10, 3 );
}