A farmer would tell you that the worst enemy of a sheep is another sheep.  In
the same way the worst enemy of a hive of bees is another hive of bees.  If
they had their way they would be a good half mile from their nearest
neighbour.  However it is very convenient for the beekeeper to have a number
of hives in one place even recognising that this means some increase of
disease and loss of crop.  I generally aim for about 3 hives to an apiary
perhaps with a few additional nucs, particularly when queen rearing.  Read Dr
Bailey's  Honey Bee Pathology for pertinent expert views on apiary size.
Chris Slade