Custom validation on array length

import { defineField, defineType } from 'sanity';

export default defineType({
  type: 'document',
  name: 'templateGallery',
  title: 'Template Gallery',
  fields: [
    // ...
    // some other fields
    // ...
    defineField({
      type: 'boolean',
      name: 'isLanding',
      title: 'Landing',
    }),
    defineField({
      type: 'array',
      name: 'gallerySlot',
      title: 'Gallery slot',
      of: [
        {
          type: 'reference',
          to: [
            {
              type: 'gallery',
            },
          ],
        },
      ],
      validation: (Rule) =>
        Rule.custom((value, { document }) => {
          if (value && value?.length > 1 && !document?.isLanding) {
            return 'This is not a landing page. You can insert only one gallery.';
          }
          return true;
        }),
    }),
  ],
});