You can use your dog's AKC registration number, or you can use any number you like and register it with NDR:
http://www.nationaldogregistry.com/ Back when you wanted a tattoo to help your dog find his way home, registration was more important. Now, I depend on the microchips and ID tags to help my dog find his way home. But the tattoo gives notice that the dog is identified. The odds are very high that if he has a tattoo, he also has a microchip. And as I said, I can instantly identify the dog as mine because I can rattle off the 10 digit number and I also have it on his ID card along with his microchip number. (See Cole's ID card is a real ID card, used to identify him, not to convince door keepers he is a service dog.)
Back in the day, they had to be tattooed with the AKC number for OFA certs. It would even indicate that on the OFA number with a T. Now they just use "PI" (permanent identification) and it can mean either a microchip or a tattoo. But in order to get certified, the dog must have one or the other.
CJ will be tattooed with the numbering system used for German working line dogs, not his AKC registration number. I'm not sure how that numbering system works, but it is a lot shorter than 10 digits, and is required for competition.