Ship's Capacity:
well there are about 875 rooms.
If you average 3 people per room that works out to 2625 passengers.
But,
each Dinning room hold 475 guests and there are 3 rooms with two seatings each that gives a maximum of 2850 guests.
But,
the Walt Disney Theater holds about 950 guests and they have two shows per night and that means there's room for only 1900 passengers.
The real limiting factor on the capacity of the ships are the number of passengers that can fit in the lifeBOATS. I believe that this number limits the Magic and Wonder to 2600 passengers each.
As to determine how many people are on any given sailing you would just have to guess (because I don't think that there is a web site that will show ALL available rooms on any given cruise). Sometimes people that have tried Travelocity or Expedia think that they see all of the available rooms for a sailing - but I'm not sure about that.