I tattoo my pups but I've never heard of cats being done till now, although I can see that pedigree cat breeders might do this, but as a kitten grows so will the tattoo and it may become illegible. The length of the tattoo depends on the registry used - one of my early dogs was PR843, so your cat's number is not particularly long. - Brett's parents were Puk and Rolline and he was born in 1984 and was the third pup born. My own pups are now registered with the National Dog Register and those are really long numbers - about 8 digits I think. But the length of the number doesn't relate to any pain involved - it is a stamp that is set up, loaded with the special ink, and clamped onto the inside flap of the ear for about 10 seconds (sometimes less time is needed) Pups are usually done from 6 weeks onwards.