Actually, the country code can be either one or two letters or a combination of one letter and one number. I am not sure about other countries, but in the U.S. the FAA normally assigns N numbers on a random basis to private operators. There are however two exceptions.
You can request a specific N number much like getting vanity plates on your car. You can also reserve blocks of N numbers. Airlines usually do this so that they can have consecutive ship numbers on their aircraft and have them match or be close to their ship/fleet number. For example, Delta's 757's are all N6XXDA, DL
or DN up to ship 699. The only exception is 690 which is registered N900PC. From ship 699 it continues as N6700 and ship number 6700, 6701, 6702, etc.
Here is a link to the FAA's registration website. There is a lookup function which can come in handy sometimes.
"I love mankind. It's people I can't stand." - Charles Shultz