Shopify has recently added a new quantity limit on items in cart.

This sets a maximum quantity per item that can be added to cart.

Shopify has automatically turned this on for all stores and it might be limiting the number of items customers can checkout with for your products.

If you sell products in large quantities, you should check this setting in Shopify.

Shopify Settings > Checkout > Add to cart limit

If you see a message like this, you need to updated this Shopify setting.