In the product page you find "Inventory" where you can add the minimum and maximum quantity of purchase.
It is optional and it is available when you need. If it is not activated, the system puts 1 automatically.
If you put the successive number you can indicate the quantity of purchase amount. I.e. minimum quantity = 1 and successive = 2, it means the user can buy 1, 3, 5 items of that product.
The maximum quantity of purchase calculates the minimum and successive quantity set up. I.e. : minimum quantity = 2, successive = 3, maximum = 4, the user buys maximum 2 items of that product >> minimum + successive (2 + 3) = it would be 5 but it has been set up 4 as maximum of quantity of purchase.
If you set up minimum = 3, successive = 3, maximum = 9, available quantity 7 the user buys maximum 6 items.
Attention! If you set up a minimum quantity, the maximum quantity has to be equal or greater than the minimum quantity.