If you're using Send Stored Codes, we would issue as many codes as the quantity ordered for that item (e.g. ordering qty=2 would issue 2 codes); at present, the Send Stored Codes feature cannot disregard the quantity ordered and just always issue one code per order, but we'll put that on the wishlist for consideration as a possible new feature at some point in the future. You can at least disable "Let Buyer Edit Quantity" in the product's settings if you only want buyers to be able to purchase one code at a time.
You might wish to consider using Send Generated Codes, where we send the order details to a script at your end that returns a code which we would forward to the buyer. Your script would determine how many codes to output based on the quantity we send it, so you could just have it always output one code regardless of quantity. This script would be quite simple if it disregards the specific order details posted to it, just treating that transmission as a trigger to output one code from a list you'd maintain at your end, so it would be easy to program and thus quite affordable if you'd need to hire a developer to program it for you; we can recommend the competent, E-junkie-experienced developers listed at the "developer directory" link at the top of this page.