Bill says:

Power requirements are determined by the power you are consuming not the length of track. How many trains are you going to be running simultaneously? Will they be sound equipped? Will there be a lot of lights or tortoises being driven by your command station? If you are going to be running the layout with a large group of people each running their own train this is a lot more than running just yourself or one other person with you. In the case of the latter you probably could get by with a 5 amp command station and a breaker for each power district.
Add up your total continuous consumption and don’t forget the switch machines and any lights being lit off track power. Sometimes you can keep your power consumption down by simply replacing light bulbs wit LED’s. Once you know the consumption divide it by your command station output and that will tell you how many power supplies you will need. A number less than 1 means your command station can handle the requirements.
The other thing you have to consider is distance from the power supply to the furthest point. There are limits but if you place the command station at the middle of the layout and run a 12 gauge bus you can probably go 50 feet in each direction without significant drop in power in the bus.
Good luck