Rats are highly sociable and wild colonies can grow into the thousands if given the opportunity and a supporting food source. 

Most pet rat keepers agree that groups of three or more are usually more stable than when rats are kept in a pair. 

It is now frowned upon to keep rats singularly and deprived of rat company. Lone rats can and do suffer from clinical depression, meaning chemical changes happen in the brain when deprived of fellow rat communication and bonds.