Current wisdom states every 10 feet. Some even install track feeders to every piece of track. It's tough to make a mistake this way. Get individual wires, not Romex that has two wires wrapped in a sheathing. This way, the two wires are readily accessible for connecting track feeders.
Solid core holds it shape better than stranded wire - making for a cleaner looking bus.
Besides that, it's dirt cheap. When calculating the size of track bus wire to use, get the next size bigger than normally would be used. It will be much less expensive to use an over size than to use an under size. Wire is cheap, so using one size larger than calculated is cheap insurance. Again, this is to insure the booster's short circuit protection will work reliably.