Short answer:
It doesn't really matter. The more stands the higher the current carrying capacity a wire can handle. Also more strands means more flexibility and less breakages.  The wires in a pinball are only required to handle a relatively small amount of current and voltage for which any wire of similar size could handle easily.  Just remember when ordering the 18 and 22 AWG stated in a pinball relates to the total cross sectional area of the wire and not of the individual strands. You would need to know the AWG of each strand x the number of strands.

Have you considered just purchasing an old pinball wiring loom? It would have all the colours you need I would think and with the correct sizing too. This is what I thought I would do if I ever needed to completely replace  any wires

Long answer:
The voltage is just a statement of what the voltage the insulation of the wire is designed and tested up to. It is not related to the current or the power that the wire can take through it. 100 volts should be fine for most pinball extra low voltage applications. The only difference with higher voltage wire is it may have thicker insulation or be made out of a different insulating material.

In Australia we typically don't use the AWG system. Wire usually comes as the number of strands/size of each strand. So 16/.30mm would be 16 strands at .3 millimetres each in diameter. If no one knows the strand size you could count the number of strands and use a vernier or similar on 1 strand and order that size or nearest to.

Tinned copper is what is used in a pinball but straight normal copper would work just fine.