It would be either “foci” or “focuses”. Both are correct. It doesn’t matter which one you use as long as you keep it consistent, but the latter is far more common.
Focus’ would be used to show possession. For example, if we have several objects that we call “focuses”:
(plural): The focus’ colors are fading.
(singular):
- The focus’ color is fading. or:
- The focus’s color is fading.
The singular version is true for names that end in “s” as well. For example, James’ and James’s are both correct. Just pick one form and stick with it.