People often get confused between layout size and power capacity. The size of your layout has an impact on the wire size used for the track bus and overall wiring practices used when building your layout. To read more about proper DCC wiring go here: http://www.wiringfordcc.com/track.htm
These examples below are for the 2 amp Power Cab starter set.
DCC Systems are selected based on your needs of locomotive capacity and operator capacity. The general rule of thumb is for HO scale locos is to use .5 amps per loco, in N-scale use .25 amps per loco. You only count the number of locos being used, not parked.
If you have a Power Cab you can enable the built in meter to see what you power usage is really like: press prog/esc 6 times, look for "set cab params" press enter, on the "show track current" screen press 1 to enable. The Powercab has a 2 amp capacity. If you are upgrading from the Power Cab to a booster the first booster MUST BE the SB5 Smartbooster!
If you have a PH-Pro 5 amp system, the only way to get an accurate reading of your power usage is with a meter specifically designed for DCC such as the RRAMPmeter. A fantastic item, it can be panel mounted or used to spot check “in the field” http://tonystrains.com/rrampmeter-by-dcc-specialties/ If you find out you need more capacity, buy a booster such as the PB5 and daisy chain the control bus. Take a look at these diagrams: https://ncedcc.zendesk.com/hc/en-us/articles/200590789-Booster-Diagrams