how do you increase your field limit when you max it?

Build terraformers on your planet. They give you five fields (one of which you should use for another terraformer).

On moons, build lunar bases. They work in the same way, except you get only 4 fields.

wait what happens if i dont have the ability to make a terraformer?

then you will need to destroy some buildings to make room. I believe this is done by clicking on the picture of the building you wish to destroy in the building menu, then on the next page it say below the description how much it cost to destroy the building and has a place to click to make it happen.

Or level up the necessary technologies, which can be found on the Technologies page. Never fill your planet to maximum unless you know for certain you don't want to build anything else.