The never ending question on how many to stock before you have your first sale. And there is a very simple answer.

Here is an email response I posted recently:
You need to stock enough shirts in a reasonable size breakdown maybe a 2-3-5-2 and see if that works.

2 = small

3 = medium

5 = large

2 =XL

That is the number of shirts per dozen you need to stock so when you run out you will run out of all sizes more or less at the same time. Then you need to carry that many shirts in every color you are going to offer in each design. Your printer will not be willing to print small quantities to order for you and it will make your operation very cost ineffective. Since you are selling online you need to have stock on hand. You will not be able to have your printer replace small numbers for you since if you don’t have it you won’t sell it.

If, for example you are carrying 10 designs and you offer them in 5 different colors and you carry a 2-3-5-2 breakdown and you order a bare minimum of 36 pieces per design/color you would order 1800 shirts. This will keep your printing costs low and you will have 6/S, 9/M, 15/L, and 6/XL on the shelf of every color/design you carry. You might also talk to your printer and see if he would be interested in signing a contract with you for a fulfillment program where he also will stock an agreed upon number in his shop that you may pull from.
This helps in the senario that if, for example on your first day in business you sell 6 small shirts in white of design 1 you would be out of stock but he would still have some in stock to send to you. You would of course be responsible for buying whatever the agreed upon number of shirts he will stock for you at the end of the contract unless you decide to renew it. – Mariellen Auer