One credit = one file download. Buy as many as you need — you get a key, enter it on the download page and pick your files.
Bonus: orders of 5 credits or more include ECU Archive Matcher free. It compares the file you downloaded against the file on your bench and tells you whether the hardware and software numbers match — so you know before you write whether the file actually belongs on that vehicle.
Please read: credits are reduced by 1–10 per month while a key is not used at all. Keys you keep using are never reduced. Use your credits within the month to keep them.
One credit = one file download