Space

Rant mode engaged.

The Martian atmosphere is about 3% nitrogen. It probably had more once, but currently there is very little nitrogen on the planet. If you were to extract 100% of the nitrogen in the already very thin martian atmosphere and fix it into organics, you could potentially create about 30cm of soil. This is assuming you don't lock any of that nitrogen into plants, people, or other organic molecules (plastics, glues, etc.). Realistically, the upper limit of martian population is about 300M before 100% of the nitrogen is used up. But this is assuming that you don't terraform.

If you terraform, you do so by dumping a large quantity of additional gas into the atmosphere. Primarily, this has will be carbon dioxide and water (it is what is available if you add heat), and maybe oxygen if you spend even more energy splitting it from on of the other two. Mars, however, has very low gravity and will lose hydrogen, methane, water and oxygen to space (this is entirely independent of the magnetic field, although the lack of magnetic field may speed this process up) due to thermal escape. Basically, the gravity on Mars is too low to have an atmosphere that warm and thick without shedding it to space over time.

Unfortunately, the 3% nitrogen will also get warmer and easier to shed. If we terraform Mars, we are actually reducing its carrying capacity for life in the long term by dumping it's rarest resource into space. Sure, this process will take thousands of years, but we could easily populate to 300M by then if we conserve this resource and use it wisely.

Mars one day will have nitrogen quotas. Like: "Sorry, you can't have a baby because we don't have enough nitrogen to make their body." And the only source will be importing it from other places in the solar system, or through ridiculous things like transmutation of oxygen in particle accelerators... If we terraform, we hit this cap earlier.

Mars under domes and in tunnels should be the goal.

Extended rant: the total load bearing capacity of the solar system will also likely hinge upon nitrogen. But we'd be in Dyson swarm territory before it becomes problematic.