"It was a great day in which we learned a lot about various enterprises created by new business people, many of WHO will certainly go on to become tomorrow’s big hitters."
"Whom". As it refers to the object of what "we learned".
In English grammar, "who" refers to the subject. Where you are looking for / referring to a noun/pronoun form. "Whom" refers to an object, an object of a verb or a preposition.
So "who" is a nominative pronoun that acts like a subject of a verb (Who are
you?) and a direct object, is the object of the verb (Whom
did you call
yesterday?), or a preposition (He is the person to whom
I gave the money).
In your mind, just think of subject/object. I hope it helps.