To a certain extent there is safety to be found in numbers. A well defended retreat of several families is less likely to be attacked than that of a single family. Only if you have the protection of total isolation -deep in the North Woods, Arizona desert, etc. – should a single family retreat be consider.
One person can only remain on guard duty so long. Even a well armed single family would be overwhelmed in a short time by the coordinated attack of only lightly armed looters. Multiple family retreats offer the following advantages:
(1) Coordinated in instituting a coordinated and well prepared defensive system that would not be possible for the single family.
(2) Sharing of the tasks of defense, hunting, food preparation, house keeping, farming, etc
(3) The potential of attracting qualified medical personnel (most important) to an established survival retreat