your server variable you need to validate is the quantity field in the form.
in this instance it is in forms, (forms are numbered from zero) and the field is cupcake1_4_Quantity_Add.
After choosing this field, click on the PLUS and you will get a popup giving you the options for minimum and maximum number.
set allow decimals to 0 and leave the allow blank entry unchecked.
after this you need to add the validation show if server behavior