Short answers, pulled from the story.
Insulae were multi-story apartment buildings that housed most citizens of ancient Rome during the early imperial period. These structures contained living spaces above ground-floor shops known as tabernae and accommodated ordinary plebeians and upper-middle class equites.
Residents included ordinary plebeians and upper-middle class equites who lacked vast wealth while traditional elites occupied single-family domus homes. Wealthy Romans such as senators owned these structures including Marcus Licinius Crassus who speculated heavily in real estate across the city.
Augustus restricted building heights to approximately 70 Roman feet after earlier towers reached nine storeys. Nero reduced this limit further following the Great Fire of Rome on the 2nd of May 64 AD before Trajan lowered the maximum height to 60 Roman feet by 17.75 meters.
The five-storey Insula dell'Ara Coeli dates from the second century AD and sits at the Capitoline Hill base. Structures at Ostia Antica reveal luxury features missing from most Roman housing examples and date from the second and third centuries AD according to archaeological evidence.
Regionary catalogues listed approximately 42,000 to 46,000 insulae in the city during the fourth century. Population figures fluctuated between 700,000 and 800,000 citizens in that era while grain requirements helped estimate total population numbers feeding Rome and surrounding areas.