Hi,
You first need to go in the menu Products>Characteristics. There, create a new characteristic with the name you want (it will be on the left of the dropdown on the product page), and then one value for each quantity.
Then, edit your product. In its settings, add the characteristic you just created in the "charactersitics" setting.
Once done, you'll have a variants tab which will appear at the top of the product edit screen.
There, you can click on the "add variants" button to add the variant for each quantity of the characteristic.
And once done, you can edit each variant to change the price, the stock, the description, etc at your own discretion.