I think one cat is fine if the owners are there with them a lot. But if the cat has to be alone a lot of time then another animal is good for them. Cat or dog. Just something to keep them company. Cats are loners but they can't be alone all the time.

My neighbors had a cat and it was mean, they got another cat and he made her mean so I think you also need time to socialize them to each other and to people. If you have children don't let them make your cat mean. Kids can be great for playing with the cat for company but if you don't have kids to chase the cat around the house to get that excercise than another cat will be great for that.

My sister's cat is getting big and chubby and it's a loner. No one to chase with. Her husband is there but when we get older we can't rough house with them like kittens want to do. They need companions to play with human or animal,