number of wires in a Wire?

Where it is important is simple. A wire needs to be able to flex easily, with a higher strand count it can bend much better and not retain any form. Some wire is also designed with a very high tolerance to heat, this is very important in your engine compartment. You do not want a high heat source melting off the insulation and shorting out the wire.

Some people will claim that the higher the strand count, the more surface area in the wire to carry current. Over the years I have played with many different types of wires, a good welding cable for power / ground worked awesome, as did car audio wire. But home ac wiring was definitely not the best to use (never used this personally) but have replaced lots of it. When you live on an isolated island (redneckville) they tend to use what they have.