There is another thread which gives the example
Je me demande combien d'entre eux vont passer les examens aujourd'hui ?
I wanted to add my question as a comment on to one of the answers but I wasn't allowed to because of not enough reputation.
What I was wondering is whether it is also possible to say only "Combien d'eux" rather than "Combien d'entre eux" or is this totally incorrect?
I ask because when I saw the title of the other question, the phrase that immediately sprung to mind was "combien d'eux" - But my French is very rusty and wasn't that good in the first place.
Thanks for any help in bringing my French back to life again!