For normal 3 rail conventional transformer powered trains, a power feeder every 2-3 feet should suffice for a large layout. A small layout may only need 2 or 3 for the entire layout.
If you are using Lionel TMCC, Legacy, or any other '0' gauge command system, the power feeders are the same as mentioned above.

If you are using DCC control for trains, then you need to read here.
Wiring is completely different.
DCC wiring for beginners- http://www.wiringfordcc.com/intro2dcc.htm
Track wiring I- http://www.wiringfordcc.com/track.htm
Track wiring II- http://www.wiringfordcc.com/track_2.htm

What manufacturer of trains are you running, and what type of track are you using?

Larry