What is a UPC-A?
What sits on groceries in the United States and Canada, and the ancestor of every barcode here - the first product ever scanned at a checkout, in 1974, carried one. Technically it is an EAN-13 whose leading digit is zero, which is why a scanner will often read a UPC-A and hand back thirteen digits with a nought on the front.
What it holds
Eleven digits, or twelve with the check digit on the end. The first digit is a number system character - 0 for most groceries, 2 for items priced by weight, 3 for pharmaceuticals - so it is not free to choose if the code has to mean something to a till.
Making a lot of them
Drop a .txt or .csv on this page and every line becomes its own UPC-A, all of them zipped together. A CSV is read from its first column. Nothing is uploaded - the codes are drawn in this page, on your machine - so a list of five thousand costs nothing and takes as long as your laptop takes.