Snippet to allow only products from a particular category to be added to the cart at a time.

allow-only-products-from-one-category.php

add_filter( 'woocommerce_add_to_cart_validation', 'wc_limit_one_per_order', 10, 2 );
function wc_limit_one_per_order( $passed_validation, $product_id ) {

$terms = wp_get_post_terms( $product_id, 'product_cat' );
foreach ( $terms as $term ) $categories[] = $term->slug;
if ( in_array( 'clothes', $categories ) ) {  // 'clothes' is the defined product category. Change this value to the slug of your choice. 
		return $passed_validation;
	}
 
/*	if ( 55 !== $product_id ) {
		return $passed_validation;
	}
*/

	if ( WC()->cart->get_cart_contents_count() >= 1 ) {
		wc_add_notice( __( 'Produts from the clothes category cannot be purchased with other products. Please, empty your cart first and then try again.', 'woocommerce' ), 'error' );
		return false;
	}

	return $passed_validation;
}