Set a universal limit for the max quantity, per product, that can be added to cart. Does not require customers to log in.
WooCommerce Max Quantity is an extremely simple extension for WooCommerce that lets you set a max limit for the number of items that can be added to the cart. It’s simple because there is only 1 option (your desired maximum number). It’s a universal limit, meaning this limit affects each product in your WooCommerce store.
The limit is per product, not per the entire cart.
This works for guest buyers, as well as logged-in buyers.
Get the plugin free at WordPress.org. Support is also available, there.
Fork it on GitHub.