I think you are confusing two things here when you compare Legacy to DCC. The TMCC/Legacy command signal has nothing to do with track power. A single connection from the base terminal to the outside (ground) rails is the only thing you need for signal.
Track power requirements are based on the number of trains you plan to run. Track feeders and wire gauge are based on the size of the layout.
If you are planning to use DCS, that is a whole different animal, one with which I zero experience (on purpose).