Well, it came up in conversation, so I had to run the numbers.
The people per square mile (p/sqm) in New York City, by the 1990 census, was about 27-28,000 p/sqm.
If you took everyone in the world, I used six point five billion people, and shipped them to an urbanized Texan Arcology, the population per square mile would be about 24,000 p/sqm. Plenty of room to stretch your arms.
Downside, everyone would have to wear black, learn some clever witticisms, and eat at overpriced sushi restaurants.
Upside, we could break up the rest of the planet for spare parts.
So, I wondered, we're overcrowded how?