In the sentence: Please call my supervisor - John Wick - on Friday.
Is an em dash used or an en dash used?

An em dash is used.

Explanation:

I know this because Hyphens are usually used for separating numbers in a phone number or connect two words, here is an example; light - hearted. I know it isn't an en dash because they are used as a substitute for the word "through", and I don't think they are trying to say "Supervisor "through" John Wick "through" on Friday." so it can't be that one either. That only leaves the em dash which is used for this purpose since it's used to break in thought, in between sentences. I know you weren't asking anything about hyphens but I wanted to state something about those too for a better understanding. Hope this helps!

i used to know but i learned it a long time ago sorry

explanation: