The triggered ability will trigger when you cast Mentor's Guidance no matter what permanents you control. As that ability tries to resolve, it will check to see if you control at least one permanent with the listed type or subtypes. If you do, Mentor's Guidance will be copied once, not once for each of those permanents. If you don't control a qualifying permanent at that time, the triggered ability won't resolve and it won't copy Mentor's Guidance.