To your question
"If I mentor one person a year for 35 years and teach each person to mentor someone else, who in turn teaches the next person to mentor, etc... how many people would I have reached in 35 years?"
I calculated that you would have 34,359,738,367 "mentees."
The following steps explain how I got that answer. After one year there would be one mentee, after two years there would be total three mentees (one mentee that you did the second year + the first year mentee + the mentee of your first mentee).
The formula that I used is as follows:
[LaTeX ERROR: Convert failed]
where n is the number of years.