Well... firstly there's just the fact that you could squeeze several billion human beings into an area the size of the Isle of Man, so if Hive worlds are anywhere near as crowded and multi-tiered as they're supposed to be, it's not hard to do the math. I can only go by the lexicanum here, but here's the following excerpt:
Sure, there's nothing inherently stopping a civilised world from being a pleasant place to live, but with adequately rational social engineering and political will, hive worlds could also be made into very livable places (as is apparently the case with the industrial centres of Ultramar. *rolls eyes*) The problem is that lack of adequately rational social engineering and political will in the imperium at large.There are probably more people on Necromunda than have ever lived in the entire history of Terra up until the end of the twentieth century. An attempted census of Trazior Hive four thousand years ago revealed an estimated population of a billion in the upper habitation levels alone - no further attempt has been made to count Necromunda's population in Trazior or any other of the several thousand hives on the planet.