First, go to the Registration section on the side-bar. Next, click the price tiers button.
Now, either editing or creating a price tier, you will see a Quantity option with a min and a max.
If you do not change anything, the result will be the default checkbox for the price tier, as seen below.
If you want a quantity range of 1 to 5, you can keep the Min value the same, but enter Max of 5. You will now see the following below on your price tier.
If you want only a quantity of 2 and only two, you must set both the Min AND the Max at 2, as seen below.
And than your price tier will look like this!