It depends how many trains your running at the same time. As already mentioned the Lionel bricks are a good way to go.
One 180w powerhouse will run 2 or 3 trains. This is a rough estimate. Long passenger trains with lights use more power. Obviously any trains where several locos are Mu'ed will use more power as well.
I run 2 trains passenger or freight along with a switching operation at the same time on 1 180w powerhouse on my 10'x16' layout without issue.
As Karl stated above. Good wiring is key. I recommend 14ga wire for bus and feeders and feeders at least every 10 feet.