What Neil says
or in other words the controler you get in a starter set is the bare min needed to work 1 loco with 2 or 3 small wagons/coaches it will run it perfectly
but buy another loco next year and try and run them together or buy another couple of wagons later and you may find that little controler will start cutting out if you turn it up to much.... maybe you might want to run lights or accesories and you will need something bigger
as a rough rule
1 small loco 1 amp is fine
average small railway maybe running 1 or 2 small locos or a large loco then 4-5 amps
10 amps and you should be able to run most combinations of anything if you need more than 10 amps you have a seriously big railway
Tony