It will be tough to work with UN population numbers. The "refugees" have a history of being born and not ever dieing. The Families get their benefits according to numbers of family members. It will be possible to determine approximate numbers by careful interview of objective (is there such) census takers. I have little doubt that the Palestinians would be ordered not to cooperate with such a census.
A small point, hal90000 sent reports of people huddling in their basements, few houses in this region have basements!