// Prevent Redirect after cart error
add_filter( 'woocommerce_cart_redirect_after_error', '__return_false');
add_action( 'woocommerce_add_to_cart_validation', 'restrict_only_one_item_in_cart' );

 function restrict_only_one_item_in_cart($cart_item_data) {

   global $woocommerce;
   $item_count = $woocommerce->cart->cart_contents_count;

   if($item_count >= 20){
      return false;
   }
   return $cart_item_data;
}

if((is_page('strongshop') || is_page('strongshopmobile')) && $item_count >= 20){
      return false;