Furnace wire specification?

I am connecting a humidstat to furnace. I would normally use furnace/thermostat wire for this purpose except that I need 8 conductors and the largest furnace/thermostat wire seems to be 5 conductors. I would like to use another 18guage 8 conductor wire but not sure if there are any specifications I need to ensure it meets before I can use it inside furnace cabinet.