If you want to allow customers to order more than 20 of an item, or decimal parts thereof you will need to remove the quantity dropdown box. In the shopping setup area, go to display options, then you can change the quantity format to be a "Free text field"
There are also other options like "Min/Max" and "Remove". The remove option assume people only every purchase a maxium quantity of 1. And so an quantity field is inappropriate.
On both combo boxes and min max, the minimum quantity, maximum quantity, quantity increment and quantity in stock are all taken into account. The only difference between MIN/MAX and COMBO is how high the combo box goes. By default max quantity is normally 0, meaning there is no theoretical limit on the quantity, so please keep that in mind... If you are using MIN/MAX, please ensure you have updated all the max fields.
Quantity increment is good if you want to sell only in fixed numbers, ie a carton of 30 products. eg you show the per unit trade price, but you only allow purchase of 30, or 60 or 90 items etc. Alternatively you can put this quantity in the unit of measure, but then you must use the carton price, not the item price.
The maximum precision for quantities are 3 decimal places. This is the same for increments, so if you sell items in kilograms, then you can enter 0.125 as a quantity or quantity increment.
The free text quantity box is best for any precise quantities with decimal parts. But note, quantity fields can only contain valid numbers, and not measurements nor spaces nor words.
The minimum box should ideally be set to 1, or a higher order quantity, or a decimal number higher than 0.