What's the difference between Advisor and Mentor?
In the context of your question the answer of Bryan Krause is almost certainly correct - no real difference. 

But in a wider context, the idea of a mentor is a bit broader than that of an advisor. An advisor will usually have some official relationship as an intermediary between a student and the university with official responsibilities. A mentor, on the other hand, can be quite informal. 

In my doctoral studies I was lucky to have both. My advisor advised me on the specific doctoral work and the field. But I had a separate mentor in a quite different mathematical field in which I was much less adept. But he was and remains my role model for what a faculty member should be and he gave me good advice and aided by development at one critical point. 

With that in mind, you normally have one advisor (exceptions exist), but you can have several mentors.