Are they all the same size or do you have groups of sizes? If they're all the same size you can make either fixed price listings or store listings with a multiple quantity.
Or if you have say 5 size Sm, 5 size Med & 5 size Large for example, make separate listings for the different sizes with whatever quantities you have. I don't mean 1 listing for all 5 but 1 listing with a quantity of 5. Bidder A can buy 1 shirt, Bidder B can buy 2 etc.
Follow?
You can do that in your store or the same in the fixed price format. Don't get confused with the 'lot' thing or it could mess up your numbers, unless you want to sell them in lots. I don't know how many you have but lots would be for say, 10 groups of 10 shirts each buyer would be buying a lot of 10.
I hope I explained that right and I didn't confuse you. :)