It's generally recommended to use the singular form for custom post type (CPT) slugs. This is because CPTs are used to represent a single item or post, rather than a group of items.
For example, if you have a CPT for "Books," it would make more sense to use the slug "book" rather than "books." This is because each individual book post will have its own URL, rather than all of the books being grouped together on a single "books" page.
Using the singular form for CPT slugs can also help to avoid confusion and make your site's URLs more intuitive and easier to understand. However, there may be cases where it makes more sense to use the plural form for your CPT slug, depending on the specific needs of your site. Ultimately, the choice of whether to use singular or plural for your CPT slug will depend on your specific use case.