"Though slavery was a prevailing feature of all Mediterranean countries in antiquity, the Romans had more slaves and depended more on them than any other people.
It is impossible, however, to put an accurate figure on the number of slaves owned by the Romans at any given period: for the early Empire with which we are concerned conditions varied from time to time and from place to place. Yet, some estimates for Rome, Italy, and the Empire are worth attempting. The largest numbers were of course in Italy and especially in the capital itself. In Rome there were great numbers in the imperial household and in the civil service - the normal staff on the aqueducts alone numbered 700 (Frontin. Aq. 116-7). Certain rich private individuals too had large numbers - as much for ostentation as for work (Sen. Ep.110.17). Pedanius Secundus, City Prefect in AD 61, kept 400 slaves (Tac. Ann. 14.43.4), Gaius Caecilius Isidorus, freedman of Gaius Caecilius, left 4116 in his will in 8 BC, while some owners had so many that a nomenclator had to be used to identify them (Pliny HN 33.135; 33.26). However, there is evidence to suggest that these cases were not typical - even for great houses. Sepulchral inscriptions for the rich noble gens the Statilii list a total of approximately 428 slaves and freedpersons from 40 BC to AD 65. When these figures are analysed, the number of slaves and freedpersons definitely owned by individual members of the gens is small, e.g. Statilius Taurus Sisenna (consul of AD 16) and his son had six, Statilius Taurus Corvinus (consul ordinarius of AD 45) had eight, and Statilia Messalina, wife of Nero, four or five. Seneca, a man of extraordinary wealth, believed he was travelling frugally when he had with him one cartload of slaves (most likely four or five) (Ep 87.2). References in Juvenal and the Scriptores Historiae Augustae suggest that many non-plebeian Romans had either no slave or merely one or two (Sat. 3.286; 9.64-67,142-7; S.H.A. Hadr.17.6). From evidence such as this Westermann, Hopkins and others are understandably cautious when attempting to come to a total figure for slaves in the city of Rome in the 1st century AD. Hopkins' estimate of 300,000-350,000 out of a population of about 900,000-950,000 at the time of Augustus seems plausible." - John Madden, Slavery in the Roman Empire Numbers and Origins