Read this: http://www.wiringfordcc.com/intro2dcc.htm
Visit NCE Layout List: http://www.ncedcc.com/#!layout-registry/crqu
Booster requirements are based on locomotive capacity needed in a given area. We only count how many locos will actually be running at the same time. In most cases we use 1/2 amp per loco in HO scale. So a 5amp booster is good for 10 locos in one location.
Power districts (blocks) are usually based on the geography of the track layout. Example: You divide the mainline up into 2 pieces (or more) for power districts. A helix is a power district. Each yard is a power district. each switching area is a power district. The general idea is to keep the rest of the layout running when something happens in a certain area.
Under DCC you can have multiple locomotives in a single location running independently. So to deal with the increase in traffic density the wiring needs to be more robust, and have short circuit protection of some sort. Read this: http://www.wiringfordcc.com/track.htm
below is a random example I found online