These sentences are not strictly equivalent and also possibly a mistranslation because passer un examen aujourd’hui doesn’t mean “pass an exam today” but “attend/take an exam today”, the former would be réussir un examen.

Anyway, the first one correctly translates “how many of them”. There is a known set of people the sentence is referring to.

The second sentence doesn’t focus on a particular set of people, the question is just about knowing the total number of people that will take it.

Of course, if the particular group of people is actually the full set, both sentences are equivalent.