Open the resource configuration.
Create a tip.
Fill in the following parameters:
1. Enter the name.
2. Specify the text of the message.
3. Choose the type of tip Error.
4. Click the Constraints tab.
Add a limit of the total shares - 2.
Then it will be impossible to book one or two tickets.
The error text is displayed on a red background when you select options and in the cart.
But the Place order button in the shopping cart is inactive.
Look in action.