I've seen some serial numbers that have been disclosed on the net and can be used to activate any number of copies of software. How is that possible? Doesn't the software provider check the serial numbers against their database, so that every code can be used limited number of times?
I can think of at least three "unlock code" scenarios used by software I have purchased.
Simple/Offline. You are issued an unlock code that is fully portable and there are no online checks. The unblock code can be validated offline; for example. if may have certain bytes that are a checksum on the remaining bytes. This offers only the barest of speedbumps to a pirate. but it does allow the publisher to figure out who gave away their unlock code and pursue legal action. Piracy, of course, is a felony, and using someone else's unlock code in a crime. This technique is used by Beyond Compare.
Challenge/Response/Offline. You install the software first, and use it to generate a "challenge" based on a fingerprint of your hardware (CPU serial number, MAC address, sometimes even hard drive serial number). The publisher will issue an unlock code that will only work on computers with the same fingerprint. This technique is used by ImageLine.
Online. You are issued a serial number. When you sign on, the system checks to see if your serial number is already active on the network, and shuts you down if it is. First person shooter/deathmatch style games often use this technique.
Sounds like in your case the first type is used, so there is no online check and you can copy and steal unlock codes as much as you want, although by doing so you are committing a crime.
You're assuming that all computers are connected to the Internet. If a computer is stand-alone, or on a private network, it will have no access to the software provider. All it can do is check that the serial number appears valid, based on some secret internal algorithm. If that's the case, the same serial number will work any number of times.