The two ways you mentioned are both correct. There are several ways to convey the meaning. 1. 사람 두 명이 더 올 거예요. (사람 두 명, two people, forms a noun phrase to function as the subject) 2. 사람이 두 명 더 올 거예요. (두 명 functions like an adverbial phrase - a bit hard to grasp grammatically but common) 3. 두 사람(이) 더 올 거예요. (두 사람 is about the same as 사람 두 명 - it probably sounds smoother) 4. 올 사람이 두 명 더 있어요. = There are two more people still to come. 5. 더 올 사람이 두 명 있어요. (same as) #4 and #5 have a slightly different focus than the others, but the broad meaning is the same. Of the rest, #3 is probably the most commonly heard, followed by #2 and then #1.

Because "사람 두 명" is kind of like the whole noun. You basically always add particles to the classifiers/counters. I know it might be somehow counterintuitive for you since in Chinese you would say 两个人 (so the order is completely different) and there are no such particles in Chinese, but well, as I always say, you just need to accept it haha.