“No-one is sure how many slaves existed in the Roman Empire. Even after Rome has passed it days of greatness, it is thought that 25% of all people in Rome were slaves. A rich man might own as many as 500 slaves and an emperor usually had more than 20,000 at his disposal.”
A logical assumption is that slaves lead poor lives simply because they were slaves. In fact, a good master looked after a good slave as an equally good replacement might be hard to acquire – or expensive. A good cook was highly prized as entertaining was very important to Rome’s elite and rich families tried to outdo each other when banquets were held – hence the importance of owning a good cook.
Those slaves who worked down mines or had no trade/skill were almost certainly less well looked after as they were easier and cheaper to replace.”
Source:History Learning Site