Searing - How much oil to use?

TL;DR: Sous-vide cooking + cast iron sear?

I'm still learning how to use cast iron. My goal with CI is to sear meats after cooking them sous-vide (after drying the excess moisture)

Here's the question: how much oil should I put in the pan? Just barely enough to coat the bottom, "swimming in it", or somewhere in between?

I'm doing a low-carb diet, so putting any kind of starch on the meats before searing won't work.