Pulling power.

[Packertrain]

How do I know how many cars I can pull with a single engine? Is as simple as whatever works?  I ask because I am doing an N scale layout of the Tahachapi Loop and some other more worldwide "challenges".
     As I am new to this (I know patience is key for the building part) I am wondering how much track one transformer will power. And, do I need one transformer per track and keep them seperated though the layout will look integrated?
     Thanks for any help. PLease email me ([email protected]) if you like.