If you list at a fixed price then you can stipulate the number of items available, these must all be the same. If you have multiple variations of the same item,  ie more than one size or orcolour,etc then you can do a multi variant listing in some catagories. If it is available in the catagory you are wanting to list on the the option will be displayed just below where it shows the catagory:  List multiple variations of your item, click on the arrow to the left and you will see the link to create variations and also a link to Learn More