I suggest developing a wire colour standard, then being sure you have enough to maintain the standard. You can run your wire colour standard by the forum to get input. This is very important, both for wire colour and wire size. an inconsistent wire colour decision can cause great confusion a few years later when you just can't remember what that wire is for. I have a wire standard, I label both ends of all my wires, and I still get confused years later. That is because my wire naming standard for some items has changed and evolved, as has my wiring methods. This is part of the hobby.
One factor is if you will be running DCS or not. With DCS, a layout of your size should have 2, 3 or 4 channels in use. For each channel, you will need a separate set of wires from the DCS AIU to a terminal strip, then wires from the terminal strip to the track. This helps reduce the amount of wire you need.
Running wires in a neat and easy to access path requires way more wire than you think. Kind of like golf, a 6500 yard course requires about 10,000 steps (assuming one step = 1 yard).
In summary:
1) develop a wire colour standard that includes wire gauge. There is track power, uncoupler power, switch power, accessory power, building lighting, DC power for some lighting or accessories, Miller signs power, etc
2) develop a wire labelling method and standard naming convention. my labels include DCS# for DCS wiring, AIU# for AIU wiring, W#T for sWitches where 'T' is the first letter of the town or district where the switch is located, U#T for uncouplers, S## for signals, etc
You can see that there is much more to wire than the track. As GunrunnerJohn said, you want to buy your wire at Home Depot or an industrial electrical supplier, where you can get a much better price. 500ft at home depot may be less than 100ft from a hobby shop. I found an electrical supplier to have a much better selection of single strand coloured wire than home depot (at least where I live).
