Trantor is stated to have a population of “forty billions”.
I haven’t found any mention of Trantor’s surface area, but if we assume it is similar to Earth’s 510,065,623 km², then that means a mere 78 people per km², or less than one person per hectare.
Even if we assume reasonable losses of land area due to infrastructure, industry, offices, public spaces, etc., but notably no land for agriculture (Trantor was fed by the output of 20 other planets), this is nowhere near enough people to populate the crowded, multi-level, planet-wide city that Asimov describes. We’re off by several orders of magnitude.
I considered he might be using long scale billions (a million million), but references to there being about 50 million people in each of the 800 districts shows he was indeed using short scale billions (a thousand million).
Is this just a matter of Asimov having written the books in the 1950s, when the Earth’s population was only 2.5 billion (less than a third of today’s nearly 8 billion) and thinking 40 sounded like enough without actually doing the math?