Q.what size pot do I need
I was just given 400mg of wildflower seed. How do I plant it in a pot?

When it comes to containers, you can choose pots, wooden window boxes, half whiskey barrels, old tires or even an old wheelbarrow. The possibilities are endless. I do not know how many seeds you have in that packet (it's variable depending on the type of seed) but if you go with pots, you are going to need larger sized pots and more than one. As for how to plant the seed, your seed packet should give you directives on that. Oftentimes with wildflower seed you are directed to simply sprinkle it on the ground with a sprinkling of soil on top.
What kind of wildflower mix do you have? This is something to think about, particularly if you are going to plant in traditional pots. When planting in pots it's best to plant wildflowers that do not exceed 18 inches tall. A mix containing tall sunflowers, for example, may not fit that bill.
I hope that helps. For more information on wildflowers in containers, please visit the following link:
https://www.gardeningknowhow.com/ornamental/flowers/fgen/container-grown-wildflowers.htm