Fork is Post Mount. Rotor is 160mm. Caliper is MT500. Which adapter do I need? Rotor bottoms out the caliper as seen in image.

You need a 20mm post mount adapter

The post mounts on that fork must be for a 140mm rotor, so as others said, you’ll need a +20mm post mount adapter.

Here is my MT500 with an 180mm rotor on a fork with 160mm post mounts. Same idea as what you’re trying to do if you need a pic for reference.

I suggest going with an actual Shimano branded post mount adapter since they tend to space Shimano calipers perfectly and don’t tend to tweak the caliper alignment as you torque the bolts down like the cheaper ones tend to do.

https://preview.redd.it/je59uw697t6g1.jpeg?width=3227&format=pjpg&auto=webp&s=6a9eaf31418736c3a35f17ccf42cd0d601a92be3