If you are running a promotion or a big sale, it's a sure thing your customers are going to grab everything that they can get their hands on. So, the way you can actually control this is by adding a product limit to their purchases. Here's how you can do it:
Go to your Dashboard > Product > View Product.
After that, click on any of the products that you would like to limit by clicking on the blue icon.
Scroll down in the settings until you find Advanced Settings and click on it.
Here, you can see that there is a setting to limit products for your customer's purchases. You can edit it according to your business needs.
Key in the limit that you want and don't forget to click Submit!