Lets take a 60km radius island where the people have access to early XXI century technology. The whole island was planned and artificially built, so it has no mountains or otherwise bad/sterile terrain.
Since it was artificially built, there are enough rivers, lakes, and the hills /valleys were pre-planned so the freshwater from the rain makes it cycle smoothly.
Also, there is no mineral production. Assume all mineral / metal needs come from asteroid mining/magic/etc. There is little to no surplus, but no scarcity, not even rare earth minerals.
Aside from that, all the food, livestock, clothing and industrialized products and energy must be produced inside the island. They can use any technology available to an early XXI century Earth (some 20 years - 2035 - ahead at most). The industry and the society is zero-waste.
And the island has no useable ocean around. So they cannot use seawater, fish or deploy boats or aquatic vehicles. Travel to other nations is rare, and is made by air.
How large a population could this secluded island nation support?