Limit Cart to a Single Item

<?php
if ( ! class_exists( 'WooCommerce' ) ) {
    return;
}

add_filter( 'woocommerce_add_to_cart_validation', 'scseed_limit_cart_one_item', 10, 2 );
function scseed_limit_cart_one_item( $passed, $product_id ) {
    if ( WC()->cart->is_empty() ) {
        return $passed;
    }
    wc_add_notice(
        __( 'Only one item is allowed in the cart at a time. The existing item has been replaced.', 'woocommerce' ),
        'notice'
    );
    WC()->cart->empty_cart();
    return $passed;
}