It is estimated that during the Roman Empire 20% to 30% of Italy's population were slaves. For the empire as a whole that share was between 10% and 15%.
In these times, even modest Roman households might expect to own two or three slaves.
Today our global economy does not depend on slavery (albeit it has not been completely eradicated), but has a different, and more invisible, dependence.
Buckminster Fuller called it "energy slaves", to describe a dependence on mostly fossil energy, expressed in energy equivalent of work done by a human worker.
Another example: a single round-trip transatlantic flight requires more energy per passenger than the passenger can generate with their own muscles over their entire life.
We take these energy slaves for granted in our daily lives. They carry us around, heat our meals, pump water to our homes, clothe us, do the cleaning and washing, delivering our messages (including this one), and so on.
Humans always used exogenous energy, i.e. energy from outside of their body. Warming near a campfire or using oxen to pull a plow are all examples of that.
What's different today is that we tapped into ancient sunlight energy in the form of coal, oil and gas.
This energy was captured and condensed over millions of years, and we're burning through it in a span of just a few hundred years.