Set WooCommerce cart product count total to be divisible by number

You can set WooCommerce cart total product count to be divisible by a certain number, if it is not divisible by that number then do not allow the user to checkout.

add_action( 'woocommerce_check_cart_items', 'check_cart_items_conditionally' );
/**
 * Check cart items conditionally displaying an error notice and avoiding checkout
 * @link https://stackoverflow.com/questions/65767078/disable-woocommerce-checkout-if-cart-items-count-is-not-a-multiple-of-n
 */
function check_cart_items_conditionally() {
    $multiple_of = 6; // <= Here set the "multiple of" number if ( WC()->cart->get_cart_contents_count() % $multiple_of != 0 ) {
        wc_add_notice( sprintf( __('You need to buy in quantities of %s products', 'woocommerce'), $multiple_of ), 'error' );
    }
}

The divisible number in the code above is 6, using the PHP modulo operator it the cart count is not divisible by 6 the error notice is displayed and the user can’t proceed to checkout until the cart amount is fixed.

Cart notice

Checkout error