Push select, then LB to see the trophies. Tells you how much you have.
You haven't rescued the fifth hostage I see. That must be the one you mean. It's after you rescue that one you need to collect 400 to get the last hostages.
Don't remember how many it is until he contact you about the fifth. Maybe 300. remember solving a riddle nets you a trophy as well. Collecting trophies as Batman or Catwoman goes towards the total count. Catwoman trophies are usually easier to obtain
since she has less gadgets. Usually just involves climbing somewhere.