It depends on how good your track is. What is the maximum number of sections between feeder and the train that will give reliable operation. Using old fashioned Lionel tubular O gauge, I like to have no track more that three sections away from a feeder. That means seven sections per feeder. For 68 feet, that is 80 sections, which means 10 to 12 feeders. I'd begin with 8 and play around with the track connections to make them tighter. With longer track sections or good tight connecting pins, fewer feeders are needed.
IF you working with something modern like Fast Track, or whatever they use now, I have no idea.
It's an art, not a science.
