Rats are social animals and need to live with at least one other rat
Rats normally live in huge extended family groups and it is unnatural for a rat to live alone
Lone rats can become depressed, withdrawn, unfriendly, aggressive, stop eating and die
Keeping a rat by itself is considered animal cruelty.
People that only want one pet are encouraged to get a Syrian hamster as in the wild they live alone.
In the past people were not aware that rats and other pets needed to live in pairs or groups, but now we know better …. although some myths still persist
If you have a baby rat, then you need to get another same sex baby immediately - hopefully one of it's siblings. This baby has been taken away from his/her mama, and all of her/his siblings …… this baby is grieving for it's family, alone, frightened - actually terrified …. and desperately wanting it's mama and all of it's brothers and sisters
Babies also need another at least one other baby to play with
People need to have at least two rats but how many more then 2 you should get depends on your circumstances ….. such as how many can you care for - vet care, medications when ill, surgery, cage size, food, etc
Please see
https://www.ratshackforum.com/threads/thinking-of-getting-a-rat-things-to-consider.32306/
Btw the Reference Thread has a lot of good info
https://www.ratshackforum.com/threads/reference-thread-read-only.35894/