If you are planning a 12x12 I would stay away from the starter sets. From what I recall you posting in treads, you are running n scale? A 12x12 is large, that is a lot of track in scale miles
I would look into a 5 amp system. Once you get your system and discover how easy it is to run a consist of engines you can easily be powering 6 to 8 engines. Two trains with 3/4 engines pulling
Then on a layout that size you'll want some of your turnouts powered through your system, this is when you start eating up your amperage.
Just encase you are doing HO with sound. 4 engines would be maxing a starter set.