Depends entirely on the tech. If all you have is bronze, about 10 million. If we mass produced greenhouses and tile the deserts etc., I've heard a trillion claimed as possible.

What part of his comment implied "unlimited growth"?

This isn't talking about 'unlimited growth.' It's talking about sustaining our current population structure or even tapering it off gracefully rather than facing the steep decline we're dealing with now.

Global population is not declining. It is projected to continue growing for at least another few decades.

Conversely, what population do you think would be appropriate? 5 billion, 1 billion, 100 million?

“Maintain humanity under 500,000,000 in perpetual balance with nature.”

Not with an attitude like that we aren't. I always preferred oneill cylinders anyway. They seem much more doable and they avoid the pesky gravity well problem and the enormous energy requirement to climb out of it.