How many wires should be in a new thermostat cable?
If you're building a new building and having new HVAC installed, I would simply put 1/2" conduit in the wall and be done. That way, you let the installer run the wire they need. I don't know that I would do this without at least consulting an HVAC installer to see where they would put the unit and the air return (typically the thermostat is as near the return as possible).

If you're going to just pre-run wire and dictate where the return will be and the HVAC unit will sit, I would run 18/5 wire if you know you will have a AC + gas heat, and 18/8 if you know you will have a heat pump (if you're in doubt, more will not hurt). Your most basic setup on most modern units needs 4 control wires plus the fifth blue "C" wire that provides power to the thermostat. It won't cover anything elaborate like a heat pump, but your greatest risk would be the installer uses your wire to pull the wire they actually need.