I am wondering in this sentence - can one use "who" instead of that
He said he knew many people that had expertise in preparing taxes and he got advice from them
Can i reword this sentence - He said he knew many people who had expertise in preparing taxes and from whom he got advice.
Yes, you can, but although I prefer "who" instead of "that" for people, I don't find your rewrite at the end of the sentence to improve its readability.
... and he got advice from them is easier to follow.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.